GIFT   OF 


c 


GENERAL  SCIENCE 


BY 

BERTHA   M.  CLARK,  PH.D. 

\  4 

HEAD   OF   THE   SCIENCE   DEPARTMENT 
WILLIAM    PENN    HIGH    SCHOOL   FOR   GIRLS,    PHILADELPHIA 


NEW  YORK  •:•  CINCINNATI  •:•  CHICAGO 

AMERICAN    BOOK    COMPANY 


COPYRIGHT,    1912,  *BY 

BERTHA  M.  CLARK. 
ENTERED  AT  STATIONERS'  HALL,  LONDON. 


CLARK'S  GENERAL  SCIENCE. 
w.  P.    i 


PREFACE 

THIS  book  is  not  intended  to  prepare  for  college  entrance 
examinations ;  it  will  not,  in  fact,  prepare  for  any  of  the 
present-day  stock  examinations  in  physics,  chemistry,  or 
hygiene,  but  it  should  prepare  the  thoughtful  reader  to  meet 
wisely  and  actively  some  of  life's  important  problems,  and 
should  enable  him  to  pass  muster  on  the  principles  and 
theories  underlying  scientific,  and  therefore  economic,  man- 
agement, whether  in  the  shop  or  in  the  home. 

We  hear  a  great  deal  about  the  conservation  of  our  natural 
resources,  such  as  forests  and  waterways ;  it  is  hoped  that 
this  book  will  show  the  vital  importance  of  the  conservation 
of  human  strength  and  health,  and  the  irreparable  loss  to 
society  of  energy  uselessly  dissipated,  either  in  idle  worry 
or  in  aimless  activity.  Most  of  us  would  reproach  ourselves 
for  lack  of  shrewdness  if  we  spent  for  any  article  more  than 
it  was  worth,  yet  few  of  us  consider  that  we  daily  expend 
on  domestic  and  business  tasks  an  amount  of  energy  far  in 
excess  of  that  actually  required.  The  farmer  who  flails  his 
grain  instead  of  threshing  it  wastes  time  and  energy ;  the 
housewife  who  washes  with  her  hands  alone  and  does  not 
aid  herself  by  the  use  of  washing  machine  and  proper  bleach- 
ing agents  dissipates  energy  sadly  needed  for  other  duties. 

239207 


4  PREFACE 

The  Chapter  on  machines  is  intended  not  only  as  a  stimu- 
lus to  the  invention  of  further  labor-saving  devices,  but  also 
as  an  eye  opener  to  those  who,  in  the  future  struggle  for 
existence,  must  perforce  go  to  the  wall  unless  they  under- 
stand how  to  make  use  of  contrivances  whereby  man's  limited 
physical  strength  is  made  effective  for  larger  tasks. 

The  Chapter  on  musical  instruments  is  more  detailed  than 
seems  warranted  at  first  sight ;  but  interest  in  orchestral 
instruments  is  real  and  general,  and  there  is  a  persistent 
desire  for  intelligent  information  relative  to  musical  instru- 
ments. The  child  of  the  laborer  as  well  as  the  child  of  the 
merchant  finds  it  possible  to  attend  some  of  the  weekly 
orchestral  concerts,  with  their  tiers  of  cheap  seats,  and  noth- 
ing adds  more  to  the  enjoyment  and  instruction  of  such  hours 
than  an  'intimate  acquaintance  with  the  leading  instruments. 
Unless  this  is  given  in  the  public  schools,  a  large  percentage 
of  mankind  is  deprived  of  it,  and  it  is  for  this  reason  that 
so  large  a  share  of  the  treatment  of  sound  has  been  devoted 
to  musical  instruments. 

The  treatment  of  electricity  is  more  theoretical  than  that 
used  in  preceding  Chapters,  but  the  subject  does  not  lend 
itself  readily  to  popular  presentation  ;  and,  moreover,  it  is 
assumed  that  the  information  and  training  acquired  in  the 
previous  work  will  give  the  pupil  power  to  understand  the 
more  advanced  thought  and  method. 

The  real  value  of  a  book  depends  not  so  much  upon  the 
information  given  as  upon  the  permanent  interest  stimulated 
and  the  initiative  aroused,  The  youthful  mind,  and  indeed 


PREFACE  5 

the  average  adult  mind  as  well,  is  singularly  non-logical  and 
incapable  of  continued  concentration,  and  loses  interest  under 
too  consecutive  thought  and  sustained  style.  For  this  reason 
the  author  has  sacrificed  at  times  detail  to  general  effect, 
logical  development  to  present-day  interest  and  facts,  and 
has  made  use  of  a  popular,  light  style  of  writing  as  well  as 
of  the  more  formal  and  logical  style  common  to  books  of 
science. 

No  claim  is  made  to  originality  in  subject  matter.  The 
actual  facts,  theories,  and  principles  used  are  such  as  have 
been  presented  in  previous  textbooks  of  science,  but  the 
manner  and  sequence  of  presentation  are  new  and,  so  far 
as  I  know,  untried  elsewhere.  These  are  such  as  in  my 
experience  have  aroused  the  greatest  interest  and  initiative, 
and  such  as  have  at  the  same  time  given  the  maximum  bene- 
fit from  the  informational  standpoint.  In  no  case,  however, 
is  mental  training  sacrificed  to  information ;  but  mental  de- 
velopment is  sought  through  the  student's  willing  and  inter- 
ested participation  in  the  actual  daily  happenings  of  the 
home  and  the  shop  and  the  field,  rather  than  through  formal 
recitations  and  laboratory  experiments. 

Practical  laboratory  work  in  connection  with  the  study  of 
this  book  is  provided  for  in  my  Laboratory  Manual  in  General 
Science,  which  contains  directions  for  a  series  of  experiments 
designed  to  make  the  pupil  familiar  with  the  facts  and  theo- 
ries discussed  in  the  textbook. 

I  have  sought  and  have  gained  help  from  many  of  the 
standard  textbooks,  new  and  old.  The  following  firms  have 


6  PREFACE 

kindly  placed  cuts  at  my  disposal,  and  have  thus  materially 
aided  in  the  preparation  of  the  illustrations :  American  Radi- 
ator Company;  Commercial  Museum,  Philadelphia;  General 
Electric  Company  ;  Hershey  Chocolate  Company  ;  Scientific 
American;  The  Goulds  Manufacturing  Company;  Victor 
Talking  Machine  Company. 

Mr.  W.  D.  Lewis,  Principal  of  the  William  Penn  High 
School,  has  read  the  manuscript  and  has  given  me  the  benefit 
of  his  experience  and  interest.  Miss  Helen  Hill,  librarian 
of  the  same  school,  has  been  of  invaluable  service  as  regards 
suggestions  and  proof  reading.  Miss  Droege,  of  the  Baldwin 
School,  Bryn  Mawr,  has  also  been  of  very  great  service. 
Practically  all  of  my  assistants  have  given  of  their  time  and 
skill  to  the  preparation  of  the  work,  but  the  list  is  too  long 

for  individual  mention. 

BERTHA   M.   CLARK. 
WILLIAM  PENN  HIGH  SCHOOL. 


CONTENTS 


CHAPTER 

I.     HEAT 


9 

II.     TEMPERATURE  AND  HEAT 27 

III.  OTHER  FACTS  ABOUT  HEAT      .        .        .  .  31 

IV.  BURNING  OR  OXIDATION 45 

V.     FOOD .60 

VI.     WATER 70 

VII.     Am '    .         .         .81 

VIII.     GENERAL  PROPERTIES  OF  GASES 95 

IX.     INVISIBLE  OBJECTS    . ,.  100 

X.     LIGHT 104 

XI.     REFRACTION 113 

XII.  PHOTOGRAPHY   ....                 ....  126 

XIII.  COLOR 134 

XIV.  HEAT  AND  LIGHT  AS  COMPANIONS 142 

XV.     ARTIFICIAL  LIGHTING 148 

XVI.     MAN'S  WAY  OF  HELPING  HIMSELF 154 

XVII.  «THE  POWER  BEHIND  THE  ENGINE 176 

XVIII.      PUMPS   AND   THEIR    VALUE   TO   MAN 187 

XIX.  THE  WATER  PROBLEM  OF  A  LARGE  CITY       .        .        .  206 

XX.  MAN'S  CONQUEST  OF  SUBSTANCES     ...        .        .        .218 

XXI.     FERMENTATION 232 

XXII.     BLEACHING 237 

XXIII.     DYEING ...  244 

7 


CONTENTS 


CHAPTER 

XXIV. 

XXV. 

XXVI. 

XXVII. 

XXVIII. 

XXIX. 

XXX. 

XXXI. 

XXXII. 

XXXIII. 

XXXIV. 

XXXV. 

INDEX 


CHEMICALS  AS  DISINFECTANTS  AND  PRESERVATIVES 
DRUGS  AND  PATENT  MEDICINES    .... 
NITROGEN  AND  ITS  RELATION  TO  PLANTS    . 

SOUND     

MUSICAL  INSTRUMENTS 

SPEAKING  AND  HEARING        .        .        .        . 

ELECTRICITY 

SOME  USES  OF  ELECTRICITY          .... 

MODERN  ELECTRICAL  INVENTIONS 

MAGNETS  AND  CURRENTS       .... 

How  ELECTRICITY  MAY  BE  MEASURED 

How  ELECTRICITY  is  MADE  ON  A  LARGE  SCALE 


PAGE 
250 

255 
26l 
266 
284 
300 
306 
3I2 
318 
328 

339 
346 

353 


GENERAL   SCIENCE 


CHAPTER    I 

HEAT 

1.  Value  of  Fire.     Every  day,  uncontrolled  fire  wipes  out 
human  lives  and  destroys  vast  amounts  of  property ;  every 
day,  fire,  controlled  and  regulated  in  stove  and  furnace,  cooks 
our  food  and  warms  our  houses.     Fire  melts  ore  and  allows 
of  the  forging  of  iron,  as  in  the  blacksmith's  shop,  and  of  the 
fashioning  of  innumerable  objects  serviceable  to  man.    Heated 
boilers  change  water  into  the  steam  which  drives  our  engines 
on  land  and  sea.     Heat  causes  rain  and  wind,  fog  and  cloud; 
heat  enables  vegetation  to  grow  and  thus  indirectly  provides 
our  food.     Whether  heat  comes  directly  from  the  sun  or  from 
artificial  sources  such  as  coal,  wood,  oil,  or  electricity,  it  is 
vitally  connected  with  our  daily  life,  and  for  this  reason  the 
facts  and  theories  relative  to  it  are  among  the  most  important 
that  can  be  studied.     Heat,  if  properly  regulated  and  con- 
trolled, would  never  be  injurious  to  man  ;  hence  in  the  follow- 
ing paragraphs  heat  will  be  considered  merely  in  its  helpful 
capacity. 

2.  General   Effect   of    Heat.      Expansion  and   Contraction. 
One  of  the  best-known  effects  of  heat  is  the  change  which 
it  causes  in  the  size  of  a  substance.     Every  housewife  knows 
that  if  a  kettle  is  filled  with  cold  water  to  begin  with,  there 
will  be  an  overflow  as  soon  as  the  water  becomes  heated. 
Heat  causes  not  only  water,  but  all  other  liquids,  to  occupy 

9 


10 


HEAT 


more  space,  or  to  expand,  and  in  some  cases  the  expansion, 
or  increase  in  size,  is  surprisingly  large.  For  example,  if 
100  pints  of  ice  water  is  heated  in  a  kettle,  the  100  pints 
will  steadily  expand  until,  at  the  boiling  point,  it  will  occupy 
as  much  space  as  104  pints  of  ice  water. 

The  expansion  of  water  can  be  easily  shown  by  heating 
a  flask  (Fig.  i)  filled  with  water  and  closed  by  a  cork  through 
which  a  narrow  tube  passes.  As  the  water  is 
heated,  it  expands  and  forces  its  way  up  the 
narrow  tube.  If  the  heat  is  removed,  the  liquid 
cools,  contracts,  and  slowly  falls  in  the  tube, 
resuming  in  time  its  original  size  or  volume.  A 
similar  observation  can  be  made  with  alcohol, 
mercury,  or  any  other  convenient  liquid. 

Not  only  liquids  are  affected  by  heat  and 
cold,  but  solids  also  are  subject  to  similar 
changes.  A  metal  ball  which  when  cool  will 
just  slip  through  a  ring  (Fig.  2) 

FIG.    i.— As  the  J 

water  becomes  will,  when  heated,  be  too  large  to 

pa^ds'and  rises   sUP  thr°USh  the  ring'     Telegraph 

in  the  narrow  and  telephone  wires  which  in  win- 
ter are  stretched  taut  from  pole  to 
pole,  sag  in  hot  weather  and  are  much  too  long. 
In  summer  they  are  exposed  to  the  fierce  rays 
of  the  sun,  become  strongly  heated,  and  expand 
sufficiently  to  sag.  If  the  wires  were  stretched 
taut  in  the  summer,  there  would  not  be  sufficient 
leeway  for  the  contraction  which  accompanies 
cold  weather,  and  in  winter  they  would  snap. 

Air  expands  greatly  when  heated  (Fig.  3),  but  since  air  is 
practically  invisible,  we  are  not  ordinarily  conscious  of  any 
change  in  it.  The  expansion  of  air  can  be  readily  shown  by 
putting  a  drop  of  ink  in  a  thin  glass  tube,  inserting  the  tube 


FIG.  2.  —  When 
the  ball  is 
heated,  it  be- 
comes too  large 
to  slip  through 
the  ring. 


EXPANSION  AND   CONTRACTION 


II 


in  the  cork  of  a  flask,  and  applying  heat  to  the  flask  (Fig.  4). 

The  ink  is  forced  up  the  tube  by  the  expanding  air.     Even 

the  warmth  of  the  hand  is  generally 

sufficient  to  cause  the  drop  to  rise 

steadily  in  the  tube.  This  means  that 

the  air  in  the  flask  occupies  more 

space  than  formerly,  and  since  the 

quantity  of  air  has  not  changed,  each 

cubic  inch  of  space  must  hold  less 

warm  air  than  it  held  of  cold  air ; 

that  is,  one  cubic  inch  of  warm  air 

weighs  less  than  one  cubic  inch  of 

cold  air,  or  warm   air  is  less  dense 

than  COld  air.       All  gases,  if  not  COn-    FIG.  3  —As  the  airing  is  heated, 

fined,  expand  when  heated  and  con-      J*  expafnds .^d  escapes  in  the 

form  of  bubbles. 

tract  as  they  cool. 

Heat,  in  general,  causes  substances  to  expand 
or  become  less  dense. 

3.   Amount   of   Expansion    and   Contraction. 
While  most  substances  expand  when  heated 
and  contract   when    cooled,  they  are  not  all 
affected  equally  by  the  same  changes  in  tem- 
perature.   Alcohol  expands  more  than  water, 
and    water  more  than  mercury.      Steel  wire 
which  measures  \  mile  on  a  snowy  day  will 
gain  25  inches  in  length  on  a  warm  summer 
day,  and  an  aluminum  wire  under  the  same 
FIG.  4. —As  the  conditions  would  gain  50  inches  in  length, 
ft^ex^ifd^lmi      4'   Advantages  an(*  Disadvantages  of  Expan- 
forces  the  drop  of  sion  and  Contraction.     We  owe  the  snug  fit  of 
lbe'     metal  tires  and  bands  to  the  expansion  and  con- 
traction resulting  from  heating  and  cooling.     The  tire  of  a 
wagon  wheel  is  made  slightly  smaller  than  the  wheel  which 


12  HEAT 

it  is  to  protect ;  it  is  then  put  into  a  very  hot  fire  and  heated 
until  it  has  expanded  sufficiently  to  slip  on  the  wheel.  As 
the  tire  cools  it  contracts  and  fits  the  wheel  closely. 

In  a  railroad,  spaces  are  usually  left  between  consecutive 
rails  in  order  to  allow  for  expansion  during  the  summer. 

The  unsightly  cracks  and  humps  in  cement  floors  are  some- 
times due  to  the  expansion  resulting  from  heat  (Fig.  5). 

Cracking  from  this 
cause  can  frequently 
be  avoided  by  cutting 
the  soft  cement  into 
squares,  the  spaces  be- 

FlG.  5.  — A  cement  walk  broken  by  expansion  due  r 

to  sun  heat.  tween  them  giving  op- 

portunity for  expansion 
just  as  do  the  spaces  between  the  rails  of  railroads. 

In  the  construction  of  long  wire  fences  provision  must  be 
made  for  tightening  the  wire  in  summer,  otherwise  great  sag- 
ging would  occur. 

Heat  plays  an  important  part  in  the  splitting  of  rocks  and 
in  the  formation  of  debris.  Rocks  in  exposed  places  are 
greatly  affected  by  changes  in  temperature,  and  in  regions 
where  the  changes  in  temperature  are  sudden,  severe,  and 
frequent,  the  rocks  are  not  able  to  withstand  the  strain  of 
expansion  and  contraction,  and  as  a  result  crack  and  split. 
In  the  Sahara  Desert  much  crumbling  of  the  rock  into  sand 
has  been  caused  by  the  intense  heat  of  the  day  followed 
by  the  sharp  frost  of  night.  The  heat  of  the  day  causes 
the  rocks  to  expand,  and  the  cold  of  night  causes  them  to 
contract,  and  these  two  forces  constantly  at  work  loosen  the 
grains  of  the  rock  and  force  them  out  of  place,  thus  producing 
crumbling. 

The  surface  of  the  rock  is  the  most  exposed  part,  and 
during  the  day  the  surface,  heated  by  the  sun's  rays,  expands 


EXPANSION  AND   CONTRACTION 


and  becomes  too  large  for 
the  interior,  and  crumbling 
and  splitting  result  from 
the  strain.  With  the  sud- 
den fall  of  temperature  in 
the  late  afternoon  and 
night,  the  surface  of  the 
rock  becomes  greatly 
'chilled  and  colder  than  the 
rock  beneath  ;  the  surface 
rock  therefore  contracts 
and  shrinks  more  than  the 
underlying  rock,"  and  again 
crumbling  results  (Fig. 
6). 

On  bare  mountains,  the 


FlG.   6.  —  Splitting    and    crumbling   of  rock 
caused  by  alternating  heat  and  cold. 


heating  and  cooling  effects  of  the  sun  are  very  striking  (Fig.  7); 

the  surface  of  many  a 
mountain  peak  is  cov- 
ered with  cracked  rock 
so  insecure  that  a  touch 
or  step  will  dislodge  the 
fragments  and  start 
them  down  the  moun- 
tain slope.  The  lower 
levels  of  mountains  are 
frequently  buried  sev- 
eral feet  under  debris 
which  has  been  formed 
in  this  way  from  higher 
peaks,  and  which  has 
slowly  accumulated  at 

FIG.  7.  —  Debris  formed  from  crumbled  rock.         the  lower  levels. 


14  HEAT 

5.  Temperature.  When  an  object  feels  hot  to  the  touch, 
we  say  that  it  has  a  high  temperature ;  when  it  feels  cold  to 
the  touch,  that  it  has  a  low  temperature;  but  we  are  not 
accurate  judges  of  heat.  Ice  water  seems  comparatively 
warm  after  eating  ice  cream,  and  yet  we  know  that  ice  water 
is  by  no  means  warm.  A  room  may  seem  warm  to  a  person 
who  has  been  walking  in  the  cold  air,  while  it  may  feel 
decidedly  cold  to  some  one  who  has  come  from  a 
warmer  room.  If  the  hand  is  cold,  lukewarm  water 
feels  hot,  but  if  the  hand  has  been  in  very  hot  water 
and  is  then  transferred  to  lukewarm  water,  the  latter 
will  seem  cold.  We  see  that  the  sensation  or  feeling 
of  warmth  is  not  an  accurate  guide  to  the  tempera- 
ture of  a  substance;  and  yet  until  1592,  one  hundred 
years  after  the  discovery  of  America,  people  relied 
solely  upon  their  sensations  for  the  measurement  of 
temperature.  Very  hot  substances  cannot  be  touched 
without  injury,  and  hence  inconvenience  as  well  as 
the  necessity  for  accuracy  led  to  the  invention  of 
the  thermometer,  an  instrument  whose  operation 
depends  upon  the  fact  that  most  substances  expand 
when  heated  and  contract  when  cooled. 

6.  The  Thermometer.  The  modern  thermometer 
consists  of  a  glass  tube  at  the  lower  end  of  which  is 
a  bulb  filled  with  mercury  or  colored  alcohol  (Fig. 
8).  After  the  bulb  has  been  filled  with  the  mer- 

FTP      8 

Making  a  cury,  it  is  placed  in  a  beaker  of  water  and  the  water 
thermom-  }s  neat.ed  by  a  Bunsen  burner.      As  the  water  be- 

eter. 

comes  warmer  and  warmer  the  level  of  the  mercury 
in  the  tube  steadily  rises  until  the  water  boils,  when  the  level 
remains  stationary  (Fig.  9).  A  scratch  is  made  on  the  tube  to 
indicate  the  point  to  which  the  mercury  rises  when  the  bulb 
is  placed  in  boiling  water,  and  this  point  is  marked  212°. 


SOME  USES   OF  A    THERMOMETER 


The  tube  is  then  removed  from  the  boiling  water,  and   after 

cooling  for  a  few  minutes,  it  is  placed  in  a  vessel  containing 

fi  nely  chopped  ice  ( Fig.  i  o).    The  mercury 

column  falls  rapidly,  but  finally  remains 

stationary,    and    at    this    level    another 

scratch  is  made  on  the  tube  and  the  point 

is  marked  32°.     The  space  between  these 

two  points,  which  represent  the  tempera- 
tures of  boiling  water  and  of  melting  ice, 

is  divided  into  180  equal  parts  called  de- 
grees.    The  thermometer  in  use  in  the 

United  States  is  marked  in  this  way  and 

is    called    the    Fahrenheit   thermometer 

after  its  designer.     Before  the   degrees 

are  etched  on  the  thermometer  the  open 
end  of  the  tube  is  sealed. 

The  Centigrade  ther- 
mometer, in  use  in  foreign 
countries  and  in  all  scien- 
tific work,  is  similar  to  the  Fahrenheit  except 
that  the  fixed  points  are  marked  100°  and  o°, 
and  the  interval  between  the  points  is  divided 
into  100  equal  parts  instead  oTf  into  1 80. 

The   boiling  point  of  water  is    212°   F.   or 
100°  C. 

The  melting  point  of  ice  is  32°  F.  or  O°  C. 

FIG    io.  — Deter-       Glass  thermometers  of  the  above  type  .are 

mining  the  lower 

fixed  point  of  a  the  ones  most  generally  used,  but  there  are 

thermometer.       maily  different  types  for  special  purposes. 

7.    Some  Uses  of  a  Thermometer.     One  of  the  chief  values 

of  a  thermometer  is  the  service  it  has  rendered  to  medicine. 

If  a  thermometer  is  held  for  a  few  minutes  under  the  tongue 

of  a  normal,  healthy  person,  the  mercury  will  rise  to  about 


FiG.  9. —  Determining  one 
of  the  fixed  points  of  a 
thermometer. 


16  HEAT 

98.4°  F.  If  the  temperature  of  the  body  regis- 
ters several  degrees  above  or  below  this  point,  a 
physician  should  be  consulted  immediately.  The 
temperature  of  the  body  is  a  trustworthy  indica- 
tor of  general  physical  condition;  hence  in  all 
hospitals  the  temperature  of  patients  is  carefully 
taken  at  stated  intervals. 

Commercially,  temperature  readings  are  ex- 
tremely important.  In  sugar  refineries  the  tem- 
perature of  the  heated  liquids  is  observed  most 
carefully,  since  a  difference  in  temperature,  how- 
ever slight,  affects  not  only  the  general  appearance 
of  sugars  and  sirups,  but  the  quality  as  well. 
The  many  varieties  of  steel  likewise  show  the  in- 
fluence which  heat  may  have  on  the  nature  of  a 
substance.  By  observation  and  tedious  experi- 
mentation it  has  been  found  that  if  hardened  steel 
is  heated  to  about  450°  F.  and  quickly  cooled,  it 
gives  the  fine  cutting  edge  of  razors ;  if  it  is  heated 
to  about  500°  F.  and  then  cooled,  the  metal  is  much 
coarser  and  is  suitable  for  shears  and  farm  imple- 
ments ;  while  if  it  is  heated  but  50°  F.  higher* 
that  is,"  to  550°  F.,  it  gives  the  fine  elastic  steel 
of  watch  springs. 

A  thermometer  could  be  put  to  good  use  in  every 
kitchen ;  the  inexperienced  housekeeper  who  can- 
not judge  of  the  "heat"  of  the  oven  would  be 
saved  bad  bread,  etc.,  if  the  thermometer  were  a 
part  of  her  equipment.  The  thermometer  can 
also  be  used  in  detecting  adulterants.  Butter 
well-made  should  melt  at  94°  (£.';  if  it  does  not,  you  may  be 
commercial  sure  that  it  is  adulterated  with  suet  or  other 

thermome-  .  .     .  .  ..       .  , 

ter.  cheap    fat.     Olive    oil   should   be   a   clear   liquid 


METHODS  OF  HEATING  BUILDINGS  17 

,.  f 
above   75°  £.  I  if>  above  this    temperature,   it    looks    cloudy,, 

you  may  be  sure  that  it  too  is  adulterated  with  fat. 

8.  Methods  of  Heating  Buildings.  Open  Fireplaces  and 
Stoves.  Before  the  time  of  stoves  and  furnaces,  man  heated 
his  modest  dwelling  by  open  fires  alone.  The  burning  logs 
gave  warmth  to  the  cabin  and  served  as  a  primitive  cooking 
agent ;  and  the  smoke  which  usually  accompanies  burning 
bodies  was  carried  away  by  means  of  the  chimney.  But  in 
an  open  fireplace  much  heat  escapes  with  the  smoke 
and  is  lost,  and  only  a 
small  portion  streams  into 
the  room  and  gives  warmth. 

When  fuel  is  placed  in 
an  open  fireplace  (Fig.  12) 
and  lighted,  the  air  im- 
mediately surrounding  the 
fire  becomes  warmer  and, 
because  of  expansion,  be- 
comes lighter  than  the  cold 
air  above.  The  cold  air,  be- 
ing heavier,  falls  and  forces 
the  warmer  air  upward, 
and  along  with  the  warm 
air  goes  the  disagreeable 
smoke.  The  fall  of  the 
colder 


FlG.   12.  —  The   open   fireplace    as    an    early 
method  of  heating. 


and  heavier  air, 
and  the  rise  of  the  warmer  and  hence  lighter  air,  is  similar  to 
the  exchange  which  takes  place  when  water  is  poured  on  oil; 
the  water,  being  heavier  than  oil,  sinks  to  the  bottom  and 
forces  the  oil  to  the  surface.  The  warmer  air  which  escapes 
up  the  chimney  carries  with  it  the  disagreeable  smoke,  and 
when  all  the  smoke  is  got  rid  of  in  this  way,  the  chimney  is 
said  to  draw  well. 

CL.    GEN.    SCI. 2 


1 8  HEAT 

As  the  air  is  heated  by  the  fire  it  expands,  and  is  pushed  up 
the  chimney  by  the  cold  air  which  is  constantly  entering  through 
loose  windows  and  doors.  Open  fireplaces  are  very  healthful 
because  the  air  which  is  driven  out  is  impure,  while  the  air 
which  rushes  in  is  fresh  and  brings  oxygen  to  the  human  being. 

But  open  fireplaces,  while  pleasant  to  look  at,  are  not  effi- 
cient for  either  heating  or  cooking.  The  possibilities  for  the 
latter  are  especially  limited,  and  the  invention  of  stoves  was  a 
great  advance  in  efficiency,  economy,  and  comfort.  A  stove  is 
a  receptacle  for  fire,  provided  with  a  definite  inlet  for  air  and  a 
definite  outlet  for  smoke,  and  able  to  radiate  into  the  room  most 
of  the  heat  produced  from  the  fire  which  burns  within. 
The  inlet,  or  draft,  admits  enough  air  to  cause  the  fire  to  burn 
brightly  or  slowly  as  the  case  may  be.  If  we  wish  a  hot  fire, 
the  draft  is  opened  wide  and  enough  air  enters  to  produce  a 
strong  glow.  If  we  wish  a  low  fire,  the  inlet  is  only  partially 
opened,  and  just  enough  air  enters  to  keep  the  fuel  smol- 
dering. 

When  the  fire  is  started,  the  damper  should  be  opened 
wide  in  order  to  allow  the  escape  of  smoke  ;  but  after  the 
fire  is  well  started  there  is  less  smoke,  and  the  damper  may 
be  partly  closed.  If  the  damper  is  kept  open,  coal  is  rapidly 
consumed,  and  the  additional  heat  passes  out  through  the 
chimney,  and  is  lost  to  use. 

9.  Furnaces.  HoJ  Air.  The  labor  involved  in  the  care  of 
numerous  stoves  is  considerable,  and  hence  the  advent  of  a 
central  heating  stove,  or  furnace,  was  a  great  saving  in 
strength  and  fuel.  A  furnace  is  a  stove  arranged  as  in 
Figure  13.  The  stove  5,  like  all  other  stoves,  has  an  inlet 
for  air  and  an  outlet  C  for  smoke ;  but  in  addition,  it  has 
built  around  it  a  chamber  in  which  air  circulates  and  is 
warmed.  The  air  warmed  by  the  stove  is  forced  upward  by 
cold  air  which  enters  from  outside.  For  example,  cold  air 


HOT   WATER  19 

constantly  entering  at  E  drives  the.  air  heated  by  6"  through 
pipes  and  ducts  to  the  rooms  to  be  heated. 

The  metal  pipes  which  convey  the  heated  air  from  the 
furnace  to  the  ducts 
are  sometimes  covered 
with  felt,  asbestos,  or 
other  non-conducting 
material  in  order  that 
heat  may  not  be  lost 
during  transmission ; 
the  ducts  which  receive 
the  heated  air  from  the 
pipes  are  built  in  the 
non-conducting  walls 
of  the  house,  and  hence 
lose  practically  no  heat. 
The  air  which  reaches 

halls       and       rooms      is    FIG.  13.  —  A  furnace.     Pipes  conduct  hot  air  to  the 

rooms. 

therefore      warm,      in 

spite  of  its  long  journey  from  the  cellar. 

Not  only  houses  are  warmed  by  a  central  heating  stove, 
but  whole  communities  sometimes  depend  upon  a  central 
heating  plant.  In  the  latter  case,  pipes  closely  wrapped  with 
a  non-conducting  material  carry  steam  long  distances  under- 
ground to  heat  remote  buildings.  Overbrook  and  Radnor, 
Pa.,  are  towns  in  which  such  a  system  is  used. 

10.  Hot  Water.  The  hot  air  which  rises  from  furnaces  often 
carries  with  it  disagreeable  dust,  and  hence  furnace  heating 
is  being  largely  supplanted  by  hot-water  heating  (Fig.  14). 
The  real  labor  involved  in  the  two  types  of  heating  is  practi- 
cally the  same,  since  coal  must  be  fed  to  the  fire  and  ashes 
must  be  removed,  but  the  hot-water  system  has  the  advantages 
of  cleanliness  and  economy.  After  the  water  in  the  radiators 


20 


HEAT 


has  become  hot  it  cools  slowly,  and  even  when  the  central 
source  of  heat  is  extinguished,   the  rooms  may  remain  quite 


FIG.  14.  —  Hot-\\ater  heating. 


HOT  WATER 


21 


warm  for  a  while.  So  long  as  the  water  in  the  radiators  is 
warmer  than  the  room  they  give  off  heat  to  the  room,  while 
in  hot-air  heating  the  extinction  of  the  central  source  of  heat 
causes  the  immediate  removal  of  the  hot-air  supply,  and  hence 
the  immediate  loss  of  heat. 

The  principle  of  hot-water  heating  is  shown  by  the  follow- 
ing simple  experiment.  Two  flasks  and  two  tubes  are 
arranged  as  in  Figure  15, 
the  upper  flask  containing 
a  colored  liquid  and  the 
lower  flask  clear  water. 
If  heat  is  applied  to  B, 
one  can  see  at  the  end 
of  a  few  seconds  the 
downward  circulation  of 
the  colored  liquid  and  the 
upward  circulation  of  the 
clear  water.  If  we  rep- 
resent a  boiler  by  B,  radi- 
ators by  the  coiled  tube, 
and  by  C,  we  shall  have 
a  very  fair  illustration  of 
the  principle  of  a  hot- 
water  heating  system. 
The  hot  water  in  the  radi- 
ators cools  and,  in  cool- 
ing, gives  up  its  heat  to 


FlG.  15.  — The  principle  of  .hot-water  heating. 


the  rooms  and  thus  warms 
them. 

This  system  does  not  ventilate  the  rooms,  since  the  radiators 
are  closed  pipes  containing  hot  water.  It  is  largely  for  this 
reason  that  thoughtful  people  are  careful  to  raise  windows  at 
intervals.  Some  systems  of  heating  secure  ventilation  by 


22 


HEAT 


confining   the  radiators  to  the  basement,  to  which  cold  air 
from  outside  is  constantly  admitted  in  such  a  way  that  it  cir- 


FlG.  16.  —  Fresh  air  from  outside  circulates  over  the  radiators  and  then  rises  into  the 
rooms  to  be  heated. 

culates  over  the  radiators  and  becomes  strongly  heated. 
This  warm  fresh  air  then  passes  through  ordinary  flues  to  the 
rooms  above,  as  in  Figures  16  and  17. 

ii.  Fresh  Air.  Fresh  air  is  essential  to  normal  healthy 
living,  and  2000  cubic  feet  of  air  per  hour  is  desirable  for 
each  individual.  If  a  gentle  breeze  is  blowing,  a  barely  per- 
ceptible opening  of  a  window  will  give  the  needed  amount, 
even  if  there  are  no  additional  drafts  of  fresh  air  into  the  room 
through  cracks.  Most  houses  are  so  loosely  constructed  that 
fresh  air  enters  imperceptibly  in  many  ways,  and  whether  we 
will  or  no,  we  receive  some  fresh  air.  This  supply  is,  how- 
ever, never  sufficient  in  itself  and  should  not  be  depended 
upon.  At  night,  or  at  any  other  time  when  gas  lights  are 
required,  the  need  for  ventilation  increases,  because  one  gas 
burner  uses  up  the  same  amount  of  air  as  four  people. 


FRESH  AIR 


24  HEAT 

111  the  preceding  Section,  we  learned  that  many  houses 
heated  by  hot  water  are  supplied  with  fresh-air  pipes  which 
admit  cold  air  into  separate  rooms  or  into  suites  of  rooms. 
In  some  cases  the  amount  which  enters  is  so  great  that  the 
air  in  a  room  is  changed  three  or  four  times  an  hour.  The 
constant  inflow  of  cold  air  and  exit  of  warm  air  necessitates 
larger  radiators  and  more  hot  water  and  hence  more  coal  to 
heat  the  larger  quantity  of  water,  but  the  additional  expense 
is  more  than  compensated  by  the  gain  in  health. 

12.  Winds  and  Currents.  The  gentlest  summer  breezes 
and  the  fiercest  blasts  of  winter  are  produced  by  the  unequal 
heating  of  air.  We  have  seen  that  the  air  nearest  to  a  stove 
or  hot  object  becomes  hotter  than  the  adjacent  air,  that  it 
tends  to  expand  and  is  replaced  and  pushed  upward  and 
outward  by  colder,  heavier  air  falling  downward.  We  have 
learned  also  that  the  moving  liquid  or  gas  carries  with  it  heat 
which  it  gradually  gives  out  to  surrounding  bodies. 

When  a  liquid  or  a  gas  moves  away  from  a  hot  object, 
carrying  heat  with  it,  the  process  is  called  convection. 

Convection  is  responsible  for  winds  and  ocean  currents, 
for  land  and  sea  breezes,  and  other  daily  phenomena. 

The  Gulf  Stream  illustrates  the  transference  of  heat  by 
convection.  A  large  body  of  water  is  strongly  heated  at  the 
equator,  and  then  moves  away,  carrying  heat  with  it  to  dis- 
tant regions,  such  as  England  and  Norway. 

Owing  to  the  shape  of  the  earth  and  its  position  with 
respect  to  the  sun,  different  portions  of  the  earth  are  un- 
equally heated.  In  those  portions  where  the  earth  is  greatly 
heated,  the  air  likewise  will  be  heated ;  there  will  be  a 
tendency  for  the  air  to  rise,  and  for  the  cold  air  from  sur- 
rounding regions  to  rush  in  to  fill  its  place.  In  this  way 
winds  are  produced.  There  are  many  circumstances  which 
modify  winds  and  currents,  and  it  is  not  always  easy  to  ex- 


CONDUCTION  25 

plain  their  direction  and  force,  but  one  very  definite  cause 
is  the  unequal  heating  of  the  surface  of  the  earth. 

13.  Conduction.  A  poker  used  in  stirring  a  fire  becomes 
hot  and  heats  the  hand  grasping  the  poker,  although  only 
the  opposite  end  of  the  poker  has  actually  been  in  the  fire. 
Heat  from  the  fire  passed  into  the  poker,  traveled  along  it, 
and  warmed  it.  When  heat  flows  in  this  way  from  a  warm 
part  of  a  body  to  a  colder  part,  the  process  is  called  conduc- 
tion. A  flatiron  is  heated  by  conduction,  the  heat  from  the 
warm  stove  passing  into  the  cold  flatiron  and  gradually  heat- 


ing it. 


In  convection,  air  and  water  circulate  freely,  carrying  heat 
with  them ;  in  conduction,  heat  flows  from  a  warm  region 
toward  a  cold  region,  but  there  is  no  apparent  motion  of  any 
kind. 

Heat  travels  more  readily  through  some  substances  than 
through  others.  All  metals  conduct  heat  well ;  irons  placed 
on  the  fire  become  heated  throughout  and  cannot  be  grasped 
with  the  bare  hand;  iron  utensils  are  frequently  made  with 
wooden  handles,  because  wood  is  a  poor  conductor  and 
does  not  allow  heat  from  the  iron  to  pass  through  it  to  the 
hand.  For  the  same  reason  a  burning  match  may  be  held 
•without  discomfort  until  the  flame  almost  reaches  the  hand. 

Stoves  and  radiators  are  made  of  metal,  because  metals 
conduct  heat  readily,  and  as  fast  as  heat  is  generated  within 
the  stove  by  the  burning  of  fuel,  or  introduced  into  the 
radiator  by  the  hot  water,  the  heat  is  conducted  through 
the  metal  and  escapes  into  the  room. 

Hot-water  pipes  and  steam  pipes  are  usually  wrapped 
with  a  non-conducting  substance,  or  insulator,  such  as  asbestos, 
in  order  that  the  heat  may  not  escape,  but  shall  be  retained 
within  the  pipes  until  it  reaches  the  radiators  within  the 
rooms. 


26 


HEAT 


The  invention  of  the  "  Fireless  Cooker  "  depended  in  part 
upon  the  principle  of  non-conduction.  Two  vessels,  one  inside 
the  other,  are  separated  by  sawdust,  asbestos,  or  other  poor  con- 
ducting material  (Fig.  18).  Foods  are  heated  in  the  usual  way 
to  the  boiling  point  or  to  a  high  temperature,  and  are  then 
placed  in  the  inner  vessel.  The  heat  of  the  food  cannot 

escape  through  the 
non-conducting  mate- 
rial which  surrounds 
it,  and  hence  remains 
in  the  food  and  slowly 
cooks  it. 

A  very  interesting 
experiment  for  the 
testing  of  the  efficacy 
of  non-conductors  may 
be  easily  performed. 
Place  hot  water  in  a 

metal  vessel,  and  note  by  means  of  a  thermometer  the  rapid- 
ity with  which  the  water  cools;  then  place  water  of  the  same 
temperature  in  a  second  metal  vessel  similar  to  the  first,  but 
surrounded  by  asbestos  or  other  non-conducting  material, 
and  note  the  slowness  with  which  the  temperature  falls. 


FIG.  18.  — A  fireless  cooker. 


CHAPTER    II 

TEMPERATURE   AND   HEAT 

14.  Temperature    not    always    a    Measure    of    the    Heat 
Present.     If  two  similar  basins  containing  unequal  quantities 
of  water  are  placed  in  the  sunshine  on  a  summer  day,  the 
smaller  quantity  of  water  will  become  quite  warm  in  a  short 
period  of  time,  while  the  larger  quantity  will  become  only 
lukewarm.     Both  vessels  receive  the  same  amount  of  heat 
from  the  sun,  but  in  one  case  the  heat  is  utilized  in  heating 
to  a  high  temperature  a  small  quantity  of  water,  while  in  the 
second  case  the  heat  is  utilized  in  warming  to  a  lower  degree 
a  larger  quantity  of  water.     Equal  amounts  of  heat  do  not 
necessarily  produce  equivalent  temperatures,  and  equal  tem- 
peratures do  not  necessarily  indicate  equal  amounts  of  heat. 
It  takes  more  heat  to  raise  a  gallon  of  water  to  the  boiling 
point  than  it  does  to  raise  a  pint  of  water  to  the  boiling  point, 
but  a  thermometer  would  register  the  same  temperature  in 
the  two  cases.     The  temperature  of  boiling  water  is  100°  C. 
whether  there  is  a  pint  of  it  or  a  gallon.     Temperature  is  in- 
dependent of  the  quantity  of  matter  present;  but  the  amount 
of  heat  contained  in  a  substance  at  any  temperature  is  not  in- 
dependent of  quantity,  being  greater  in  the  larger  quantity. 

15.  The  Unit  of  Heat.      It  is  necessary  to  have  a  unit  of 
heat  just  as  we  have  a  unit  of  length,  or  a  unit  of  mass,  or  a 
unit  of  time.     One  unit  of  heat  is  called  a  calorie,  and  is  the 
amount  of  heat  which  will  change  the  temperature  of  I  gram 
of  water  i°  C.;  it  is  the  amount  of  heat  given  out  by  I  gram 
of  water  when  its  temperature  falls   i°  C.,  or  the  amount  of 
heat  absorbed  by  I  gram  of  water  when  its  temperature  rises 

27 


28  TEMPERATURE  AND  HEAT 

i°  C.  If  400  grams  of  water  are  heated  from  o°  to  5°  C., 
the  amount  of  heat  which  has  entered  the  water  is  equivalent 
to  5  X4OO  or  2000  calories;  if  200  grams  of  water  cool  from 
25°  to  20°  C.,  the  heat  given  out  by  the  water  is  equivalent 
to  5  X2OO  or  1000  calories. 

1 6.  Some  Substances  Heat  more  readily  than  Others.    If  two 
equal  quantities  of  water  are  exposed  to  the  sun  for  the  same 
length  of  time,  the  temperatures  will  be  the  same.     If,  how- 
ever, equal  quantities  of  different  substances  are  exposed,  the 
temperatures  resulting  from  the  heating  will  not  necessarily 
be  the  same.     If  a  basin  containing  mercury  is  put  on  the  fire, 
side  by  side  with  a   basin  containing  an  equal   quantity  of 
water,  the  temperatures  of  the  two  substances  will  vary  greatly 
at  the  end  of  a  short  time.     The  mercury  will  have  a  far  higher 
temperature    than  the  water,  in  spite  of  the  fact  that  the 
amount  of  mercury  is  as  great  as  the  amount  of  water  and 
that  the  heat  received  from  the  fire  has  been  the  same  in  each 
case.     Mercury  is  not  so  difficult  to  heat  as  water;  less  heat 
being  required  to  raise  its  temperature  i°  than  is  required  to 
raise  the  temperature  of  an  equal  quantity  of  water  i°.    In  fact, 
mercury  is  30  times  as  easy  to  heat  as  water,  and  it  requires 
only  one  thirtieth  as  much  fire  to  heat  a  given  quantity  of 
mercury  i°  as  to  heat  the  same  quantity  of  water  i°. 

17.  Specific  Heat.     We  know  that  different  substances  are 
differently  affected  by  heat.     Some  substances,  like  water, 
change  their  temperature  slowly  when  heated ;    others,  like 
mercury,  change  their  temperature  very  rapidly  when  heated. 
The  number  of  calories  needed  by  I  gram  of  a  substance  in 
order  that  its  temperature  may  be  increased  i°C.  is  called 
the  specific  heat  of  a  substance ;  or,  specific  heat  is  the  num- 
ber of  calories  given  out  by  I  gram  of  a  substance  when  its 
temperature  falls  i°C.      For  experiments  on  the  determina- 
tion of  specific  heat,  see  Laboratory  Manual. 


SOURCES  OF  HEAT  29 

Water  has  the  highest  specific  heat  of  any  known  sub- 
stance except  hydrogen ;  that  is,  it  requires  more  heat  to 
raise  the  temperature  of  water  a  definite  number  of  degrees 
than  it  does  to  raise  the  temperature  of  an  equal  amount  of 
any  other  substance  the  same  number  of  degrees.  Practi- 
cally this  same  thing  can  be  stated  in  another  way :  Water  in 
cooling  gives  out  more  heat  than  any  other  substance  in  cool- 
ing through  the  same  number  of  degrees.  For  this  reason 
water  is  used  in  foot  warmers  and  in  hot-water  bags.  If 
a  copper  lid  were  used  as  a  foot  warmer,  it  would  give  the 
feet  only  .095  as  much  heat  as  water;  a  lead  weight  only 
.031  as  much  heat  as  water.  Flatirons  are  made  of  iron  be 
cause  of  the  relatively  high  specific  heat  of  iron.  The 
flatiron  heats  slowly  and  cools  slowly,  and,  because  of  its 
high  specific  heat,  not  only  supplies  the  laundress  with  con- 
siderable heat,  but  eliminates  for  her  the  frequent  changing 
of  the  flatiron. 

1 8.  Water  and  Weather.    About  four  times  as  much  heat  is 
required  to  heat  a  given  quantity  of  water  one  degree  as  to 
heat  an  equal  quantity  of  earth.     In  summer,  when  the  rocks 
and  the  sand  along  the  shore  are  burning  hot,  the  ocean  and 
lakes  are  pleasantly  cool,  although  the  amount  of  heat  pres- 
ent in  the  water  is  as  great  as  that  present  in  the  earth.     In 
winter,  long  after  the  rocks  and  sand  have  given  out  their 
heat  and  have  become  cold,  the  water  continues  to  give  out 
the  vast  store  of  heat  accumulated  during  the  summer.     This 
explains  why  lands  situated  on  or  near  large  bodies  of  water 
usually  have  less  variation  in  temperature  than  inland  regions. 
In  the  summer  the  water  cools  the  region ;  in  the  winter,  on 
the  contrary,  the  water  heats  the  region,  and  hence  extremes 
of  temperature  are  practically  unknown. 

19.  Sources  of  Heat.     Most  of  the  heat  which  we  enjoy 
and  use  we  owe  to  the  sun.     The  wood  which  blazes  on  the 


30  TEMPERATURE  AND  HEAT 

hearth,  the  coal  which  glows  in  the  furnace,  and  the  oil  which 
burns  in  the  stove  owe  their  existence  to  the  sun. 

Without  the  warmth  of  the  sun  seeds  could  not  sprout  and 
develop  into  the  mighty  trees  which  yield  firewood.  Even 
coal,  which  lies  buried  thousands  of  feet  below  the  earth's 
surface,  owes  its  existence  in  part  to  the  sun.  Coal  is  simply 
buried  vegetation, —  vegetation  which  sprouted  and  grew  under 
the  influence  of  the  sun's  warm  rays.  Ages  ago  trees  and 
bushes  grew  "  thick  and  fast,"  and  the  ground  was  always 
covered  with  a  deep  layer  of  decaying  vegetable  matter.  In 
time  some  of  this  vast  supply  sank  into  the  moist  soil  and 
became  covered  with  mud.  Then  rock  formed,  and  the  rock 
pressed  down  upon  the  sunken  vegetation.  The  constant 
pressure,  the  moisture  in  the  ground,  and  heat  affected  the 
underground  vegetable  mass,  and  slowly  changed  it  into  coal. 

The  buried  forest  and  thickets  were  not  all  changed  into 
coal.  Some  were  changed  into  oil  and  gas.  Decaying  ani- 
mal matter  was  often  mixed  with  the  vegetable  mass.  When 
the  mingled  animal  and  vegetable  matter  sank  into  moist 
earth  and  came  under  the  influence  of  pressure,  it  was  slowly 
changed  into  oil  and  gas. 

The  heat  of  our  bodies  comes  from  the  foods  which  we  eat. 
Fruits,  grain,  etc.,  could  not  grow  without  the  warmth  and 
the  light  of  the  sun.  The  animals  which  supply  our  meats 
likewise  depend  upon  the  sun  for  light  and  warmth. 

The  sun,  therefore,  is  the  great  source  of  heat ;  whether  it 
is  the  heat  which  comes  directly  from  the  sun  and  warms  the 
atmosphere,  or  the  heat  which  comes  from  burning  coal, 
wood,  and  oil. 


CHAPTER    III 

OTHER   FACTS   ABOUT   HEAT 

20.  Boiling.  Heat  absorbed  in  Boiling.  If  a  kettle  of 
water  is  placed  above  a  flame,  the  temperature  of  the  water 
gradually  increases,  and  soon  small  bubbles  form  at  the  bot- 
tom of  the  kettle  and  begin  to  rise  through  the  water.  At 
first  the  bubbles  do  not  get  far  in  their  ascent,  but  disappear 
before  they  reach  the  surface ;  later,  as  the  water  gets  hotter 
and  hotter,  the  bubbles  become  larger  and  more  numerous, 
rise  higher  and  higher,  and  finally  reach  the  surface  and 
pass  from  the  water  into  the  air  ;  steam  comes  from  the  vessel, 
and  the  water  is  said  to  boil.  The  temperature  at  which  a 
liquid  boils  is^  called  the  boiling  point. 

While  the  water  is  heating,  the  temperature  steadily  rises, 
but  as  soon  as  the  water  begins  to  boil  the  thermometer  read- 
ing becomes  stationary  and  does  not  change,  no  matter  how 
hard  the  water  boils  and  in  spite  of  the  fact  that  heat  from 
the  flame  is  constantly  passing  into  the  water. 

If  the  flame  is  removed  from  the  boiling  water  for  but  a 
second,  the  boiling  ceases ;  if  the  flame  is  replaced,  the  boil- 
ing begins  again  immediately.  Unless  heat  is  constantly 
supplied,  water  at  the  boiling  point  cannot  be  transformed 
into  steam. 

The  number  of  calories  which  imist  be  supplied  to  i  gram  of 
water  at  tJie  boiling  point  in  order  to  change  it  into  steam  at 
the  same  temperature  is  called  tJie  Jieat  of  vaporization  ;  it  is 
the  heat  necessary  to  change  I  gram  of  water  at  the  boiling 
point  into  steam  of  the  same  temperature. 

31 


32  OTHER  FACTS  ABOUT  HEAT 

21.  The  Amount  of  Heat  Absorbed.     The  amount  of  heat 
which  must  be  constantly  supplied  to  water  at  the  boiling  point 
in  order  to  change  it  into  steam  is  far  greater  than  we  realize. 
If  we   put  a  beaker  of   ice   water   (water  at  o°  C.)  over  a 
steady  flame,  and  note  (i)  the  time  which  elapses  before  the 
water  begins  to  boil,  and  (2)  the  time  which  elapses  before 
the  boiling  water  completely  boils  away,  we  shall  see  that  it 
takes  about  %\  times  as  long  to  change  water  into  steam  as  it 
does  to  change  its  temperature  from  o°  C.  to  100°  C.     Since, 
with  a  steady  flame,  it  takes  5^  times  as  long  to  change  water 
into  steam  as  it  does  to  change  its  temperature  from  o°  C. 
to  the  boiling  point,  we  conclude  that  it  takes  5|  times  as 
much  heat  to  convert  water  at  the  boiling  point  into  steam  as 
it  does  to  raise  it  from  the  temperature  of  ice  water  to  that  of 
boiling  water. 

The  amount  of  heat  necessary  to  raise  the  temperature  of 
I  gram  of  water  i°  C.  is  equal  to  I  calorie,  and  the  amount 
necessary  to  raise  the  temperature  100°  C.  is  equal  to  100 
calories  ;  hence  the  amount  of  heat  necessary  to  convert 
I  gram  of  water  at  the  boiling  point  into  steam  at  that  same 
temperature  is  equal  to  approximately  525  calories.  Very 
careful  experiments  show  the  exact  heat  of  vaporization  to 
be  536.1  calories.  (See  Laboratory  Manual.) 

22.  General   Truths.     Statements  similar  to  the  above  hold 
for  other  liquids  and  for  solutions.     If  milk  is  placed  upon  a 
stove,  the  temperature  rises  steadily  until  the  boiling  point  is 
reached  ;  further  heating  produces,  not  a  change  in  tempera- 
ture, but  a  change  of  the  liquid  into  steam.     As  soon  as  the 
milk,  or  in  fact  any  liquid  food,  comes  to  a  boil,  the  gas  flame 
should  be  lowered  until  only  an  occasional  bubble  forms,  be- 
cause so  long  as  any  bubbles  form  the  temperature  is  that  of 
the  boiling  point,  and  further  heat  merely  results  in  waste  of 
fuel. 


PRACTICAL   APPLICATION  33 

We  find  by  experiment  that  every  liquid  has  its  own  spe- 
cific boiling  point ;  for  example,  alcohol  boils  at  87°  C.  and 
brine  at  103°  C.  Both  specific  heat  and  the  heat  of  vaporiza- 
tion vary  with  the  liquid  used. 

23-  Condensation.  If  one  holds  a  cold  lid  in  the  steam  of 
boiling  water,  drops  of  water  gather  on  the  lid ;  the  steam  is 
cooled  by  contact  with  the  cold  lid  and  condenses  into  water. 
Bottles  of  water  brought  from  a  cold  cellar  into  a  warm  room 
become  covered  with  a  mist  of  fine  drops  of  water,  because 
the  moisture  in  the  air,  chilled  by  contact  with  the  cold 
bottles,  immediately  condenses  into  drops  of  water.  Glasses 
filled  with  ice  water  show  a  similar  mist. 

In  Section  21,  we  saw  that  536  calories  are  required  to 
change  I  gram  of  water  into  steam  ;  if,  now,  the  steam  in 
turn  condenses  into  water,  it  is  natural  to  expect  a  release  of 
the  heat  used  in  transforming  water  into  steam.  Experiment 
shows  not  only  that  vapor  gives  out  heat  during  condensation, 
but  that  the  amount  of  heat  thus  set  free  is  exactly  equal  to 
the  amount  absorbed  during  vaporization.  (See  Laboratory 
Manual.) 

We  learn  that  the  heat  of  vaporization  is  the  same  whether 
it  is  considered  as  the  heat  absorbed  by  I  gram  of  water  in 
its  change  to  steam,  or  as  the  heat  given  out  by  I  gram  of 
steam  during  its  condensation  into  water. 

24.  Practical  Application.  We  understand  now  the  value  of 
steam  as  a  heating  agent.  Water  is  heated  in  a  boiler  in  the 
cellar,  and  the  steam  passes  through  pipes  which  run  to  the 
various  rooms ;  there  the  steam  condenses  into  water  in  the  radi- 
ators, each  gram  of  steam  setting  free  536  calories  of  heat. 
When  we  consider  the  size  of  the  radiators  and  the  large  num- 
ber of  grams  of  steam  which  they  contain,  and  consider  fur- 
ther that  each  gram  in  condensing  sets  free  536  calories,  we 
understand  the  ease  with  which  buildings  are  heated  by  steam. 

CL.    GKN.    SCI. 3 


34  OTHER   FACTS  ABOUT  HEAT 

Most  of  us  have  at  times  profited  by  the  heat  of  condensa- 
tion. In  cold  weather,  when  there  is  a  roaring  fire  in  the 
range,  the  water  frequently  becomes  so  hot  that  it  "steams" 
out  of  open  faucets.  If,  at  such  times,  the  hot  water  is  turned 
on  in  a  small  cold  bathroom,  and  is  allowed  to  run  until  the 
tub  is  well  filled,  vapor  condenses  on  windows,  mirrors,  and 
walls,  and  the  cold  room  becomes  perceptibly  warmer.  The 
heat  given  out  by  the  condensing  steam  passes  into  the  sur- 
rounding air  and  warms  the  room. 

There  is,  however,  another  reason  for  the  rise  in  tempera- 
ture. If  a  large  pail  of  hot  soup  is  placed  in  a  larger  pail  of 
cold  water,  the  soup  will  gradually  cool  and  the  cold  water 
will  gradually  become  warmer.  A  red-hot  iron  placed  on  a 
stand  gradually  cools,  but  warms  the  stand.  A  hot  body 
loses  heat  so  long  as  a  cooler  body  is  near  it ;  the  cold  object 
is  heated  at  the  expense  of  the  warmer  object,  and  one  loses 
heat  and  the  other  gains  heat  until  the  temperature  of  both  is 
the  same.  Now  the  hot  water  in  the  tub  gradually  loses  heat 
and  the  cold  air  of  the  room  gradually  gains  heat  by  con- 
vection, but  the  amount  given  the  room  by  convection  is 
relatively  small  compared  with  the  large  amount  set  free  by 
the  condensing  steam. 

25.  Distillation.  If  impure,  muddy  water  is  boiled,  drops 
of  water  will  collect  on  a  cold  plate  held  in  the  path  of  the 
steam,  but  the  drops  will  be  clear  and  pure.  When  impure 
water  is  boiled,  the  steam  from  it  does  not  contain  any  of  the 
impurities  because  these  are  left  behind  in  the  vessel.  If  all 
the  water  were  allowed  to  boil  away,  a  layer  of  mud  or  of 
other  impurities  would  be  found  at  the  bottom  of  the  vessel. 
Because  of  this  fact,  it  is  possible  to  purify  water  in  a  very 
simple  way.  Place  over  a  fire  a  large  kettle  closed  except  for 
a  spout  which  is  long  enough  to  reach  across  the  stove  and 
dip  into  a  bottle.  As  the  liquid  boils,  steam  escapes  through 


EVAPORATION 


35 


the  spout,  and  on  reaching  the  cold  bottle  condenses  and 
drops  into  the  bottle  as  pure  water.  The  impurities  remain 
behind  in  the  kettle.  Water  freed  from  impurities  in  this  way 
is  called  distilled  water,  and  the  process  is  called  distillation 
(Fig.  19).  By  this  method,  the  salt  water  of  the  ocean  may  be 
separated  into  pure 
drinking  water  and 
salt,  and  many  of  the 
large  ocean  liners 
distill  from  the  briny 
deep  all  the  drinking 
water  used  on  their 
ocean  voyages. 

Commercially,  distil- 
lation is  a  very  impor- 
tant process.  Turpen- 
tine, for  example,  is 
made  by  distilling  the 
sap  of  pine  trees.  In- 
cisions are  cut  in  the 
bark  of  the  long-leaf  pine  trees,  and  these  serve  as  chan- 
nels for  the  escape  of  crude  resin.  This  crude  liquid  is 
collected  in  barrels  and  taken  to  a  distillery,  where  it  is 
distilled  into  turpentine  and  rosin.  The  turpentine  is  the 
product  which  passes  off  as  steam,  and  the  rosin  is  the  mass 
left  in  the  boiler  after  the  distillation  of  the  turpentine. 

26.  Evaporation.  If  a  stopper  is  left  off  a  cologne  bottle, 
the  contents  of  the  bottle  will  slowly  evaporate ;  if  a  dish  of 
water  is  placed  out  of  doors  on  a  hot  day,  evaporation  occurs 
very  rapidly.  The  liquids  which  have  disappeared  from  the 
bottle  and  the  dish  have  passed  into  the  surrounding  air  in  the 
form  of  vapor.  In  Section  20,  we  saw  that  water  could  not 
pass  into  vapor  without  the  addition  of  heat ;  now  the  heat 


water  tanie 


FIG.  19.  —  In  order- that  the  steam  which  passes 
through  the  coiled  tube  may  be  quickly  cooled  and 
condensed,  cold  water  is  made  to  circulate  around 
the  coil.  The  condensed  steam  escapes  at  w. 


36  OTHER  FACTS  ABOUT  HEAT 

necessary  for  the  evaporation  of  the  cologne  and  water  was 
taken  from  the  air,  leaving  it  slightly  cooler.  If  wet  hands 
are  not  dried  with  a  towel,  but  are  left  to  dry  by  evaporation, 
heat  is  taken  from  the  hand  in  the  process,  leaving  a  sensa- 
tion of  coolness.  Damp  clothing  should  never  be  worn,  be- 
cause the  moisture  in  it  tends  to  evaporate  at  the  expense  of 
the  bodily  heat,  and  this  undue  loss  of  heat  from  the  body 
produces  chills.  After  a  bath  the  body  should  be  well  rubbed, 
otherwise  evaporation  occurs  at  the  expense  of  heat  which 
the  body  cannot  ordinarily  afford  to  lose. 

Evaporation  is  a  slow  process  occurring  at  all  times ;  it  is 
hastened  during  the  summer,  because  of  the  large  amount  of 
heat  present  in  the  atmosphere.  Many  large  cities  make  use 
of  the  cooling  effect  of  evaporation  to  lower  the  temperature 
of  the  air  in  summer;  streets  are  sprinkled  not  only  to  lay 
the  dust,  but  in  order  that  the  surrounding,  air  may  be  cooled 
by  the  evaporation  of  the  water. 

Some  thrifty  housewives  economize  by  utilizing  the  cooling 
effects  of  evaporation.  Butter,  cheese,  and  other  foods  sensi- 
tive to  heat  are  placed  in  porous  vessels  wrapped  in  wet 
cloths.  Rapid  evaporation  of  the  water  from  the  wet  cloths 
keeps  the  contents  of  the  jars  cool,  and  that  without  expense 
other  than  the  muscular  energy  needed  for  wetting  the  cloths 
frequently. 

27.  Rain,  Snow,  Frost,  Dew.  The  heat  of  the  sun  causes 
constant  evaporation  of  the  waters  of  oceans,  rivers,  streams, 
and  marshes,  and  the  water  vapor  set  free  by  evaporation 
passes  into  the  air,  which  becomes  charged  with  vapor  or  is 
said  to  be  humid.  Constant,  unceasing  evaporation  of  our 
lakes,  streams,  and  pools  would  mean  a  steady  decrease  in 
the  supply  of  water  available  for  daily  use,  if  the  escaped  water 
were  all  retained  by  the  atmosphere  and  lost  to  the  earth. 
But  although  the  escaped  vapor  mingles  with  the  atmosphere, 


RAIN,   SNOW,   FROST,   D£W  37 

hovering  near  the  earth's  surface,  or  rising  far  above  the 
level  of  the  mountains,  it  does  not  remain  there  permanently. 
When  this  vapor  meets  a  cold  wind  or  is  chilled  in  any  way, 
condensation  takes  place,  and  a  mass  of  tiny  drops  of  water 
or  of  small  particles  of  snow  is  formed.  When  these  drops 
or  particles  become  large  enough,  they  fall  to  the  earth  as 
rain  or  snow,  and  in  this  way  the  earth  is  compensated  for 
the  great  loss  of  moisture  due  to  evaporation.  Fog  is  formed 
when  vapor  condenses  near  the  surface  of  the  earth,  and  when 
the  drops  are  so  small  that  they  do  not  fall  but  hover  in  the 
air,  the  fog  is  said  '''  not  to  lift "  or  "  not  to  clear." 

If  ice  water  is  poured  into  a  glass,  a  mist  will  form  on  the 
outside  of  the  glass.  This  is  because  the  water  vapor  in  the 
air  becomes  chilled  by  contact  with  the  glass  and  condenses. 
Often  leaves  and  grass  and  sidewalks  are  so  cold  that  the 
water  vapor  in  the  atmosphere  condenses  on  them,  and  we 
say  a  heavy  dew  has  formed.  If  the  temperature  of  the  air 
falls  to  the  freezing  point  while  the  dew  is  forming,  the  vapor 
is  frozen  and  frost  is  seen  instead  of  dew. 

The  daily  evaporation  of  moisture  into  the  atmosphere 
keeps  the  atmosphere  more  or  less  full  of  water  vapor;  but 
the  atmosphere  can  hold  only  a  definite  amount  of  vapor  at 
a  given  temperature,  and  as  soon  as  it  contains  the  maximum 
amount  for  that  temperature,  further  evaporation  ceases.  If 
clothes  are  hung  out  on  a  damp,  murky  day  they  do  not 
dry,  because  the  air  contains  all  the  moisture  it  can  hold, 
and  the  moisture  in  the  clothes  has  no  chance  to  evaporate. 
When  the  air  contains  all  the  moisture  it  can  hold,  it  is  said 
to  be  saturated,  and  if  a  slight  fall  in  temperature  occurs 
when  the  air  is  saturated,  condensation  immediately  begins 
in  the  form  of  rain,  snow,  or  fog.  If,  however,  the  air  is 
not  saturated,  a  fall  in  temperature  may  occur  without 
producing  precipitation.  The  temperature  at  which  air  is 


38  OTHER  FACTS  ABOUT  HEAT 

saturated    and    condensation     begins    is    called     the    dew 
point. 

28.  How  Chills  are  Caused.     The  discomfort  we  feel  in  an 
overcrowded  room  is  partly  due  to  an  excess  of  moisture  in 
the  air,  resulting  from   the   breathing   and    perspiration    of 
many  persons.     The  air  soon  becomes  saturated  with  vapor 
and  cannot  take  away  the  perspiration  from  our  bodies,  and 
our  clothing  becomes  moist  and  our  skin  tender.     When  we 
leave  the  crowded  "tea"  or  lecture  and  pass  into  the  colder, 
drier,  outside  air,  clothes  and  skin  give  up  their  load  of  mois- 
ture through  sudden  evaporation.     But  evaporation  requires 
heat,  and  this  heat  is  taken  from  our  bodies,  and  a  chill  results. 

Proper  ventilation  would  eliminate  much  of  the  physical 
danger  of  social  events ;  fresh,  dry  air  should  be  constantly 
admitted  to  crowded  rooms  in  order  to  replace  the  air  satu- 
rated by  the  breath  and  perspiration  of  the  occupants. 

29.  Weather  Forecasts.     When  the  air  is  near  the  satura- 
tion point,  the  weather  is  oppressive  and  is  said  to  be  very 
humid.     For  comfort  and  health,  the  air  should  be  about 
two  thirds  saturated.     The  presence  of  some  water  vapor  in 
the  air  is  absolutely  necessary  to  animal  and  plant  life.     In 
desert  regions  where  vapor  is  scarce  the  air  is  so  dry  that 
throat  trouble  accompanied  by  disagreeable  tickling  is  preva- 
lent; fallen  leaves  become  so  dry  that  they  crumble  to  dust; 
plants  lose  their  freshness  and  beauty. 

The  likelihood  of  rain  or  frost  is  often  determined  by  tem- 
perature and  humidity.  If  the  air  is  near  saturation  and  the 
temperature  is  falling,  it  is  safe  to  predict  bad  weather,  be- 
cause the  fall  of  temperature  will  probably  cause  rapid  con- 
densation, and  hence  rain.  If,  however,  the  air  is  not  near 
the  saturation  point,  a  fall  in  temperature  will  not  necessarily 
produce  bad  weather. 

The  measurement  of  humidity  is  of  far  wider  importance 


HEAT  NEEDED   TO   MELT  SUBSTANCES  39 

than  the  mere  forecasting  of  local  weather  conditions.  The 
close  relation  between  humidity  and  health  has  led  many 
institutions,  such  as  hospitals,  schools,  and  factories,  to  regu- 
late the  humidity  of  the  atmosphere  as  carefully  as  they  do 
the  temperature.  Too  great  humidity  is  enervating,  and  not 
conducive  to  either  mental  or  physical  exertion  ;  on  the  other 
hand,  too  dry  air  is  equally  harmful.  In  summer  the  humid- 
ity conditions  cannot  be  well  regulated,  but  in  winter,  when 
houses  are  artificially  heated,  the  humidity  of  a  room  can  be 
increased  by  placing  pans  of  water  near  the  registers  or  on 
radiators. 

30.  Heat  Needed  to  Melt  Substances.  If  a  spoon  is  placed 
in  a  vessel  of  hot  water  for  a  few  seconds  and  then  removed, 
it  will  be  warmer  than  before  it  was  placed  in  the  hot  water. 
If  a  lump  of  melting  ice  is  placed  in  the  vessel  of  hot  water 
and  then  removed,  the  ice  will  not  be  warmer  than  before, 
but  there  will  be  less  of  it.  The  heat  of  the  water  has  been 
used  in  melting  the  ice,  not  in  changing  its  temperature. 

If,  on  a  bitter  cold  day,  a  pail  of  snow  is  brought  into  a 
warm  room  and  a  thermometer  is  placed  in  the  snow,  the 
temperature  rises  gradually  until  32°  F.  is  reached,  when  it 
becomes  stationary,  and  the  snow  begins  to  melt.  If  the  pail 
is  put  on  the  fire,  the  temperature  still  remains  32°  F.,  but  the 
snow  melts  more  rapidly.  As  soon  as  all  the  snow  is  com- 
pletely melted,  however,  the  temperature  begins  to  rise  and 
rises  steadily  until  the  water  boils,  when  it  again  becomes  sta- 
tionary and  remains  so  during  the  passage  of  water  into  vapor. 

We  see  that  heat  must  be  supplied  to  ice  at  o°  C.  or  32° 
F.  in  order  to  change  it  into  water,  and  further,  that  the 
temperature  of  the  mixture  does  not  rise  so  long  as  any  ice  is 
present,  no  matter  how  much  heat  is  supplied.  The  amount 
of  heat  necessary  to  melt  I  gram  of  ice  fe  easily  calculated. 
(See  Laboratory  Manual.) 


40  OTHER  FACTS  ABOUT  HEAT 

Heat  must  be  supplied  to  ice  to  melt  it.  On  the  other  hand, 
water,  in  freezing,  loses  heat,  and  the  amount  of  heat  lost  by 
freezing  water  is  exactly  equal  to  the  amount  of  heat  absorbed 
by  melting  ice. 

The  number  of  units  of  heat  required  to  melt  a  unit  mass 
of  ice  is  called  the  licat  of  fusion  of  water. 

31.  Climate.    Water,  in  freezing,  loses  heat,  even  though  its 
temperature  remains  at  o°  C.     Because  water  loses  heat  when 
it  freezes,  the  presence  of  large  streams  of  water  greatly  in- 
fluences the  climate  of  a  region.      In  winter  the  heat  from 
the  freezing  water  keeps  the  temperature  of  the  surrounding 
air  higher  than  it  would  naturally  be,   and  consequently  the 
cold  weather  is  less  severe.     In  summer  water  evaporates, 
heat  is  taken  from  the  air,  and  consequently  the  warm  weather 
is  less  intense. 

32.  Molding  of  Glass  and  Forging  of  Iron.     The  fire  which 
is  hot  enough  to  melt  a  lump  of  ice  may  not  be  hot  enough 
to  melt  an  iron  poker  ;  on  the  other  hand,  it  may  be  suffi- 
ciently hot  to  melt  a  tin  spoon.     Different  substances  melt, 
or  liquefy,  at  different  temperatures ;  for  example,  ice  melts 
at  o°  C,  and  tin  at  233°  C.,  while  iron  requires  the  relatively 
high  temperature  of  1200°  C.     Most  substances  have  a  definite 
melting  or  freezing  point  which  never  changes  so  long  as  the 
surrounding  conditions  remain  the  same. 

But  while  most  substances  have  a  definite  melting  point, 
some  substances  do  not.  If  a  glass  rod  is  held  in  a  Bunsen 
burner,  it  will  gradually  grow  softer  and  softer,  and  finally  a 
drop  of  molten  glass  will  fall  from  the  end  of  the  rod  into 
the  fire.  The  glass  did  not  suddenly  become  a  liquid  at  a 
definite  temperature ;  instead  it  softened  gradually,  and  then 
melted.  While  glass  is  in  the  soft,  yielding,  pliable  state,  it 
is  molded  into  dishes,  bottles,  and  other  useful  objects,  such  as 
lamp  shades,  globes,  etc.  (Fig.  20).  If  glass  melted  at  a  definite 


STRANGE  BEHAVIOR  OF   WATER  41 

temperature,  it  could  not  be  molded  in  this  way.  Iron  acts 
in  a  similar  manner,  and  because  of  this  property  the  black- 
smith can  shape  his  horseshoes,  and  the  machinist  can  make 
his  engines  and  other  articles  of  daily  service  to  man. 


FIG.  20.  —  Molten  glass  being  rolled  into  a  form  suitable  for  window  panes. 

33.  Strange  Behavior  of  Water.  One  has  but  to  remember 
that  bottles  of  water  burst  when  they  freeze,  and  that 
ice  floats  on  water  like  wood,  to  know  that  water  expands 
on  freezing  or  on  solidifying.  A  quantity  of  water  which 
occupies  100  cubic  feet  of  space  will,  on  becoming  ice,  need 
109  cubic  feet  of  space.  On  a  cold  winter  night  the  water 
sometimes  freezes  in  the  water  pipes,  and  the  pipes  burst. 
Water  is  very  peculiar  in  expanding  on  solidification,  be- 
cause most  substances  contract  on  solidifying  ;  gelatin  and 
jelly,  for  example,  contract  so  much  that  they  shrink  from  the 
sides  of  the  dish  which  contains  them. 

If  water  contracted  in  freezing,  ice  would  be  heavier  than 


42  OTHER  FACTS  ABOUT  HEAT 

water  and  would  sink  in  ponds  and  lakes  as  fast  as  it  formed, 
and  our  streams  and  ponds  would  become  masses  of  solid  ice, 
killing  all  animal  and  plant  life.  But  the  ice  is  lighter  than 
water  and  floats  on  top,  and  animals  in  the  water  beneath  are 
as  free  to  live  and  swim  as  they  were  in  the  warm  sunny 
days  of  summer.  The  most  severe  winter  cannot  freeze  a 
deep  lake  solid?  and  in  the  coldest  weather  a  hole  made  in 
the  ice  will  show  water  beneath  the  surface.  Our  ice  boats 
cut  and  break  the  ice  of  the  river,  and  through  the  water 
beneath  our  boats  daily  ply  their  way  to  and  fro,  independent 
of  winter  and  its  blighting  blasts. 

While  most  of  us  are  familiar  with  the  bursting  of  water 
pipes  on  a  cold  night,  few  of  us  realize  the  influence  which 
freezing  water  exerts  on  the  character  of  the  land  around  us. 

Water  sinks  into  the  ground  and,  on  the  approach  of 
winter,  freezes,  expanding  about  one  tenth  of  its  volume  ;  the 
expanding  ice  pushes  the  earth  aside,  the  force  in  some  cases 
being  sufficient  to  dislodge  even  huge  rocks.  In  the  early 
days  in  New  England  it  was  said  by  the  farmers  that  "rocks 
grew,"  because  fields  cleared  of  stones  in  the  fall  became 
rock  covered  with  the  approach  of  spring;  the  rocks  and  stones 
hidden  underground  and  unseen  in  the  fall  were  forced  to  the 
surface  by  the  winter's  expansion.  We  have  all  seen  fence 
posts  and  bricks  pushed  out  of  place  because  of  the  heaving 
of  the  soil  beneath  them.  Often  householders  must  re-lay 
their  pavements  and  walks  because  of  the  damage  done  by 
freezing  water. 

The  most  conspicuous  effect  of  the  expansive  power  of  freez- 
ing water  is  seen  in  rocky  or  mountainous  regions  (Fig.  21). 
Water  easily  finds  entrance  into  the  cracks  and  crevices  of 
the  rocks,  where  it  lodges  until  frozen ;  then  it  expands  and 
acts  like  a  wedge,  widening  cracks,  chiseling  off  edges,  and 
even  breaking  rocks  asunder.  In  regions  where  frequent 


HEAT  NECESSARY  TO  DISSOLVE  A  SUBSTANCE     43 


frosts  occur,  the  destructive  action  of  water  works  constant 
changes  in  the  appearance  of  the  land ;  small  cracks  and 
crevices  are  enlarged,  mas- 
sive rocks  are  pried  up  out 
of  position,  huge  slabs  are 
split  off,  and  particles 
large  and  small  are  forced 
from  the  parent  rock.  The 
greater  part  of  the  debris 
and  rubbish  brought  down 
from  the  mountain  slopes 
by  the  spring  rains  owes  its 
origin  to  the  fact  that  water 
expands  when  it  freezes. 

34.  Heat  Necessary  to 
Dissolve  a  Substance.  It 
requires  heat  to  dissolve 
any  substance,  just  as  it 
requires  heat  to  change 
ice  to  water.  If  a  handful 
of  common  salt  is  placed  in  a  small  cup  of  water  and  stirred 
with  a  thermometer,  the  temperature  of  the  mixture  falls  sev- 
eral degrees.  This  is  just  what  one  would  expect,  because 
the  heat  needed  to  liquefy  the  salt  must  come  from  some- 
where, and  naturally  it  comes  from  the  water,  thereby  lower- 
ing the  temperature  of  the  water.  We  know  very  well  that 
potatoes  cease  boiling  if  a  pinch  of  salt  is  put  in  the  water ; 
this  is  because  the  temperature  of  the  water  has  been  lowered 
by  the  amount  of  heat  necessary  to  dissolve  the  salt. 

Let  some  snow  or  chopped  ice  be  placed  in  a  vessel  and 
mixed  with  one  third  its  weight  of  coarse  salt ;  if  then  a  small 
tube  of  cold  water  is  placed  in  this  mixture,  the  water  in  the  test 
tube  will  freeze  immediately.  As  soon  as  the  snow  and  salt 


FIG.  21.  —  The  destruction  caused  by  freezing 
water. 


44  OTHER  FACTS  ABOUT  HEAT 

are  mixed  they  melt.  The  heat  necessary  for  this  comes  in 
part  from  the  air  and  in  part  from  the  water  in  the  test  tube, 
and  the  water  in  the  tube  becomes  in  consequence  cold  enough 
to  freeze.  But  the  salt  mixture  does  not  freeze  because  its 
freezing  point  is  far  below  that  of  pure  water.  The  use  of 
salt  and  ice  in  ice-cream  freezers  is  a  practical  application  of 
this  principle.  The  heat  necessary  for  melting  the  mixture 
of  salt  and  ice  is  taken  from  the  cream  which  thus  becomes 
cold  enough  to  freeze. 


CHAPTER   IV 

BURNING  OR  OXIDATION 

35.  Why  Things  Burn.  The  heat  of  our  bodies  comes 
from  the  food  we  eat ;  the  heat  for  cooking  and  for  warming 
our  houses  comes  from  coal.  The  production  of  heat  through 
the  burning  of  coal,  or  oil,  or  gas,  or  wood,  is  called  com- 
bustion. Combustion  cannot  occur  without  the  presence  of 
a  substance  called  oxygen,  which  exists  rather  abundantly 
in  the  air ;  that  is,  one  fifth  of  our  atmosphere  consists  of 
this  substance  which  we  call  oxygen.  We  throw  open  our 
windows  to  allow  fresh  air  to  enter,  and  we  take  walks  in 
order  to  breathe  the  pure  air  into  our  lungs.  What  we  need 
for  the  energy  and  warmth  of  our  bodies  is  the  oxygen  in 
the  air.  Whether  we  burn  gas  or  wood  or  coal,  the  heat 
which  is  produced  comes  from  the  power  which  these  various 
substances  possess  to  combine  with  oxygen.  We  open  the 
draft  of  a  stove  that  it  may  "  draw  well "  :  that  it  may  secure 
oxygen  for  burning.  We  throw  a  blanket  over  burning  ma- 
terial to  smother  the  fire  :  to  keep  oxygen  away  from  it.  Burn- 
ing, or  oxidation,  is  combining  with  oxygen,  and  the  more 
oxygen  you  add  to  a  fire,  the  hotter  the  fire  will  burn,  and  the 
faster.  The  effect  of  oxygen  on  combustion  may  be  clearly 
seen  by  thrusting  a  smoldering  splinter  into  a  jar  containing 
oxygen  ;  the  smoldering  splinter  will  instantly  flare  and  blaze, 
while  if  it  is  removed  from  the  jar,  it  loses  its  flame  and  again 

45 


46  BURNING   OR   OXIDATION 

burns  quietly.     Oxygen  for  this  experiment  can  be  produced 
in  the  following  way. 

36.  How  to  Prepare  Oxygen.  Mix  a  small  quantity  of  potas- 
sium chlorate  with  an  equal  amount  of  manganese  dioxide 
and  place  the  mixture  in  a  strong  test  tube.  Close  the 
mouth  of  the  tube  with  a  one-hole  rubber  stopper  in  which 
is  fitted  a  long,  narrow  tube,  and  clamp  the  test  tube  to  an 
iron  support,  as  shown  in  Figure  22.  Fill  the  trough  with 


FIG.  22.  —  Preparing  oxygen  from  potassium  chlorate  and  manganese  dioxide. 

water  until  the  shelf  is  just  covered  and  allow  the  end  of  the 
delivery  tube  to  rest  just  beneath  the  hole  in  the  shelf. 
Fill  a  medium-sized  bottle  with  water,  cover  it  with  a  glass 
plate,  invert  the  bottle  in  the  trough,  and  then  remove  the 
glass  plate.  Heat  the  test  tube  very  gently,  and  when  gas 
bubbles  out  of  the  tube,  slip  the  bottle  over  the  opening  in 
the  shelf,  so  that  the  tube  runs  into  the  bottle.  The  gas  will 
force  out  the  water  and  will  finally  fill  the  bottle.  When  all 
the  water  has  been  forced  out,  slip  the  glass  plate  under  the 
mouth  of  the  bottle  and  remove  the  bottle  from  the  trough. 
The  gas  in  the  bottle  is  oxygen. 

Everywhere  in  a  large  city  or  in  a  small  village,  smoke  is 
seen,  indicating  the  presence  of  fire  ;  hence  there  must  exist  a 
'large  supply  of  oxygen  to  keep  all  the  fires  alive.  The  supply 


SAFETY  MATCHES  47 

of  oxygen  needed  for  the  fires  of  the  world  comes  largely  from 
the  atmosphere. 

37.  Matches.     The  burning  material   is  ordinarily  set   on 
fire  by  matches,  thin   strips  of  wood  tipped  with  sulphur  or 
phosphorus,  or  both.     Phosphorus  can  unite  with  oxygen,  at 
a   fairly    low    temperature,    and    if    phosphorus    is   rubbed 
against  a  rough  surface,  the  friction  produced  will  raise  the 
temperature  of  the  phosphorus  to  a  point  where  it  can  com- 
bine with  oxygen.     The  burning  phosphorus  kindles  the  wood 
of  the  match,  and  from  the  burning  match  the  fire  is  kindled. 
If  you  want  to  convince  yourself  that  friction  produces  heat, 
rub  a  cent  vigorously   against  your  coat  and  note  that  the 
cent  becomes  warm.     Matches  have  been  in  use  less  than  a 
hundred   years.     Primitive    man    kindled    his    camp   fire   by 
rubbing  pieces  of   dry  wood   together  until  they   took  fire, 
and  this  method  is  said  to  be  used  among  some   isolated 
distant   tribes   at   the    present    time.      A    later    and    easier 
way  was  to  strike  flint  and  steel  together  and  to  catch  the 
spark  thus  produced  on  tinder  or  dry  fungus.     Within  the 
memory  of  some  persons  now  living,  the  tinder  box  was  a 
valuable    asset    to    the    home,    particularly   in    the    pioneer 
regions  of  the. West. 

38.  Safety  Matches.      Ordinary  phosphorus,  while  excel- 
lent as  a  fire-producing  material,  is    dangerously  poisonous, 
and  those  to  whom  the  dipping  of  wooden  strips  into  phos- 
phorus is   a  daily  occupation  suffer  with  a  terrible  disease 
which  usually  attacks  the  teeth  and  bones  of  the  jaw.     The 
teeth  rot  and  fall  out,  abscesses  form,  and  bones  and  flesh 
begin  to  decay ;  the  only  way  to  prevent  the  spread  of  the 
disease  is  to  remove  the  affected  bone,  and  in  some  instances 
it  has  been  necessary  to  remove  the  entire  jaw.     Then,  too, 
matches  made  of  yellow  or  white  phosphorus  ignite  easily, 
and,  when  rubbed  against  any  rough  surface,  are  apt  to  take 


48  BURNING   OR   OXIDATION 

fire.  Many  destructive  fires  have  been  started  by  the  ac- 
cidental friction  of  such  matches  against  rough  surfaces. 

For  these  reasons  the  introduction  of  the  so-called  safety 
match  was  an  important  event.  When  common  phosphorus, 
in  the  dangerous  and  easily  ignited  form,  is  heated  in  a  closed 
vessel  to  about  2 50°  C,  it  gradually  changes  to  a  harmless 
red  mass.  The  red  phosphorus  is  not  only  harmless,  but  it  is 
difficult  to  ignite,  and,  in  order  to  be  ignited  by  friction,  must 
be  rubbed  on  a  surface  rich  in  oxygen.  The  head  of  a  safety 
match  is  coated  with  a  mixture  of  glue  and  oxygen-contain- 
ing compounds  ;  the  surface  on  which  the  match  is  to  be 
rubbed  is  coated  with  a  mixture  of  red  phosphorus  and  glue, 
to  which  finely  powdered  glass  is  sometimes  added  in  order 
to  increase  the  friction.  Unless  the  head  of  the  match  is 
rubbed  on  the  prepared  phosphorus  coating,  ignition  does  not 
occur,  and  accidental  fires  are  avoided. 

Various  kinds  of  safety  matches  have  been  manufactured 
in  the  last  few  years,  but  they  are  somewhat  more  expensive 
than  the  ordinary  form,  and  hence  manufacturers  are  reluctant 
to  substitute  them  for  the  cheaper  matches.  Some  foreign 
countries,  such  as  Switzerland,  prohibit  the  sale  of  the 
dangerous  type,  and  it  is  hoped  that  the  United  States  will 
soon  follow  the  lead  of  these  countries  in  demanding  the  sale 
of  safety  matches  only. 

39.  Some  Unfamiliar  Forms  of  Burning.  While  most  of 
us  think  of  burning  as  a  process  in  which  flames  and  smoke 
occur,  there  are  in  reality  many  modes  of  burning  accom- 
panied by  neither  flame  nor  smoke.  Iron,  for  example, 
burns  when  it  rusts,  because  it  slowly  combines  with  the 
oxygen  of  the  air  and  is  transformed  into  new  substances. 
When  the  air  is  dry,  iron  does  not  unite  with  oxygen,  but 
when  moisture  is  present  in  the  air,  the  iron  unites  with  the 
oxygen  and  turns  into  iron  rust.  The  burning  is  slow  and  un- 


SOME   UNFAMILIAR  FORMS   OF  BURNING  49 

accompanied  by  the  fire  and  smoke  so  familiar  to  us,  but  the 
process  is  none  the  less  burning,  or  combination  with  oxygen. 
Burning  which  is  not  accompanied  by  any  of  the  appearances 
of  ordinary  burning  is  known  as  oxidation. 

The  tendency  of  iron  to  rust  lessens  its  efficiency  and 
value,  and  many  devices  have  been  introduced  to  prevent 
rusting.  A  coating  of  paint  or  varnish  is  sometimes  applied 
to  iron  in  order  to  prevent  contact  with  air.  The  galvanizing 
of  iron  is  another  attempt  to  secure  the  same  result ;  in  this 
process  iron  is  dipped  into  molten  zinc,  thereby  acquiring  a 
coating  of  zinc,  and  forming  what  is  known  as  galvanized 
iron.  Zinc  does  not  combine  with  oxygen  under  ordinary  cir- 
cumstances, and  hence  galvanized  iron  is  immune  from  rust. 

Decay  is  a  process  of  oxidation  ;  the  tree  which  rots  slowly 
away  is  undergoing  oxidation,  and  the  result  of  the  slow 
burning  is  the  decomposed  matter  which  we  see  and  the 
invisible  gases  which  pass  into  the  atmosphere.  The  log 
which  blazes  on  our  hearth  gives  out  sufficient  heat  to  warm 
us  ;  the  log  which  decays  in  the  forest  gives  out  an  equivalent 
amount  of  heat,  but  the  heat  is  evolved  so  slowly  that  we  are 
not  conscious  of  it.  Burning  accompanied  by  a  blaze  and 
intense  heat  is  a  rapid  process ;  burning  unaccompanied  by 
fire  and  appreciable  heat  is  a  slow,  gradual  process,  requiring 
days,  weeks,  and  even  long  years  for  its  completion. 

Another  form  of  oxidation  occurs  daily  in  the  human  body. 
In  Section  35  we  saw  that  the  human  body  is  an  engine 
whose  fuel  is  food ;  the  burning  of  that  food  in  the  body 
furnishes  the  heat  necessary  for  bodily  warmth  and  the 
energy  required  for  thought  and  action.  Oxygen  is  essential 
to  burning,  and  the  food  fires  within  the  body  are  kept  alive 
by  the  oxygen  taken  into  the  body  at  every  breath  by  the 
lungs.  We  see  now  one  reason  for  an  abundance  of  fresh 
air  in  daily  life. 

CL.    GEN.    SCI. — 4 


5O  BURNING   OR   OXIDATION 

40.  How  to  Breathe.  Air,  which  is  essential  to  life  and 
health,  should  enter  the  body  through  the  nose  and  not 
through  the  mouth.  The  peculiar  nature  and  arrangement 
of  the  membranes  of  the  nose  enable  the  nostrils  to  clean, 
and  warm,  and  moisten  the  air  which  passes  through  them 
to  the  lungs.  Floating  around  in  the  atmosphere  are  dust 
particles  which  ought  not  to  get  into  the  lungs.  The  nose 
is  provided  with  small  hairs  and  a  moist  inner  membrane 
which  serve  as  filters  in  removing  solid  particles  from  the 
air,  and  in  thus  purifying  it  before  its  entrance  into  the 
lungs. 

In  the  immediate  neighborhood  of  three  Philadelphia  high 
schools,  having  an  approximate  enrollment  of  over  8000 
pupils,  is  a  huge  manufacturing  plant  which  day  and  night 
pours  forth  grimy  smoke  and  soot  into  the  atmosphere  which 
must  supply  oxygen  to  this  vast  group  of  young  lives.  If  the 
vital  importance  of  nose  breathing  is  impressed  upon  these 
young  people,  the  harmful  effect  of  the  foul  air  may  be  greatly 
lessened,  the  smoke  particles  and  germs  being  held  back  by 
the  nose  filters  and  never  reaching  the  lungs.  If,  however, 
this  principle  of  hygiene  is  not  brought  to  their  attention, 
the  dangerous  habit  of  breathing  through  the  open,  or  at  least 
partially  open,  mouth  will  continue,  and  objectionable  matter 
will  pass  through  the  mouth  and  find  a  lodging  place  in  the 
lungs. 

There  is  another  very  important  reason  why  nose  breath- 
ing is  preferable  to  mouth  breathing.  The  temperature  of 
the  human  body  is  approximately  98°  F.,  and  the  air  which 
enters  the  lungs  should  not  be  far  below  this  temperature. 
If  air  reaches  the  lungs  through  the  nose,  its  journey  is  rela- 
tively long  and  slow,  and  there  is  opportunity  for  it  to  be 
warmed  before  it  reaches  the  lungs.  If,  on  the  other  hand, 
air  passes  to  the  lungs  by  way  of  the  mouth,  the  warming 


HOW  TO  BUILD  A   FIRE 


process  is  brief  and  insufficient,  and  the  lungs  suffer  in  con- 
sequence. Naturally,  the  gravest  danger  is  in  winter. 

41.  Cause  of  Mouth  Breathing.     Some  people  find  it  diffi- 
cult to  breathe  through  the  nostrils  on  account  of  growths, 
called  adenoids,  in  the  nose.     If  you  have  a  tendency  toward 
mouth   breathing,   let  a  physician   examine  your   nose   and 
throat. 

Adenoids  not  only  obstruct  breathing  and  weaken  the 
whole  system  through  lack  of  adequate  air,  but  they  also 
press  upon  the  blood  ves- 
sels  and  nerves  of  the 
head  and  interfere  with 
normal  brain  development. 
Moreover,  they  interfere  in 
many  cases  with  the  hear- 
ing, and  in  general  hinder 
activity  and  growth.  The 
removal  of  adenoids  is 
simple,  and  carries  with  it 
only  temporary  pain  and 
no  danger.  Some  physi- 
cians claim  that  the 
growths  disappear  in  later 
years,  but  even  if  that  is 

true,  the  physical  and  mental  development  of  earlier  years 
is  lost,  and  the  person  is  backward  in  the  struggle  for  life 
and  achievement. 

42.  How  to  Build  a  Fire.     Substances  differ  greatly  as  to 
the  ease  with  which  they  may  be  made  to  burn  or,  in  tech- 
nical terms,  with  which   they   may  be   made   to   unite  with 
oxygen.     For  this  reason,  we  put  light  materials,  like  shav- 
ings, chips,  and  paper,  on  the  grate,  twisting  the  latter  and 
arranging  it  so  that  air  (oxygen  in  the  air)  can  reach  a  large 


FIG.  23.  —  Intelligent  expression  is  often  lack- 
ing in  children  with  adenoid  growths. 


52  BURNING   OR   OXIDATION 

surface;  upon  this  we  place  small  sticks  of  wood,  piling  them 
across  each  other  so  as  to  allow  entrance  for  the  oxygen  ;  and 
finally  upon  this  we  place,  our  hard  wood  or  coal. 

The  coal  and  the  large  sticks  cannot  be  kindled  with  a 
match,  but  the  paper  and  shavings  can,  and  these  in  burning 
will  heat  the  large  sticks  until  they  take  fire  and  in  turn 
kindle  the  coal. 

43.  Spontaneous    Combustion.      We    often    hear    of    fires 
"  starting  themselves,"  and  sometimes  the  statement  is  true. 
If  a  pile  of  oily  rags  is  allowed  to  stand  for  a  time,  the  oily 
matter  will  begin  tolcombine  slowly  with  oxygen  and  as  a 
result  will  give  off  her&.     The  heat  thus  given  off  is  at  first 
insufficient  to  kindle  a  fire ;  but  as  the  heat  is  retained  and 
accumulated,  the  temperature  rises,  and  finally  the  kindling 
point  is  reached  and  the  whole  mass  bursts  into  flames.     For 
safety's  sake,  all  oily  cloths   should  be  burned  or  kept  in 
metal  vessels. 

44.  The  Treatment  of  Burns.     In  spite  of  great  caution, 
burns  from  fires,  steam,  or  hot  water  do  sometimes  occur,  and 
it  is  well  to  know  how  to  relieve  the  suffering  caused  by  them 
and  how  to  treat  the  injury  in  order  to  insure  rapid  healing. 

Burns  are  dangerous  because  they  destroy  skin  and  thus  open 
up  an  entrance  into  the  body  for  disease  germs,  and  in  addi- 
tion because  they  lay  bare  nerve  tissue  which  thereby  be- 
comes irritated  and  causes  a  shock  to  the  entire  system. 

In  mild  burns,  where  the  skin  is  not  broken  but  is  merely 
reddened,  an  application  of  moist  baking  soda  brings  imme- 
diate relief.  If  this  substance  is  not  available,  flour  paste, 
lard,  sweet  oil,  or  vaseline  may  be  used. 

In  more  severe  burns,  where  blisters  are  formed,  the  blisters 
should  be  punctured  with  a  sharp,  sterilized  needle  and  allowed 
to  discharge  their  watery  contents  before  the  above  remedies 
are  applied. 


DANGER   OF  CARBON-  DIOXIDE  53 

In  burns  severe  enough  to  destroy  the  skin,  disinfection  of 
the  open  wound  with  weak  carbolic  acid  or  hydrogen  peroxide 
is  very  necessary.  After  this  has  been  done,  a  soft  cloth 
soaked  in  a  solution  of  linseed  oil  and  limewater  should  be 
applied  and  the  whole  bandaged.  In  such  a  case,  it  is  im- 
portant not  to  use  cotton  batting,  since  this  sticks  to  the  rough 
surface  and  causes  pain  when  removed. 

45.  Carbon  Dioxide.     A  Product  of  Burning.     When  any 
fuel,  such  as  coal,  gas,  oil,  or  wood,  burns,  it  sends  forth  gases 
into  the  surrounding  atmosphere.     These  gases,  like  air,  are 
invisible,  and  were  unknown  to  us  for  a  long  time.     The  chief 
gas  formed  by  a  burning  substance  is  called  carbon  dioxide 
(CO2)  because  it  is  composed  of  one  part  of  carbon  and  two 
parts  of  oxygen.     This  gas  has  the  distinction  of  being  the 
most  widely  distributed  gaseous  compound  of  the  entire  world ; 
it  is  found  in  the  ocean  depths  and  on  the  mountain  heights, 
in  brilliantly  lighted  rooms,  and  most  abundantly  in  manu- 
facturing towns  where  factory  chimneys  constantly  pour  forth 
hot  gases  and  smoke. 

Wood  and  coal,  and  in  fact  all  animal  and  vegetable  mat- 
ter, contain  carbon,  and  when  these  substances  burn  or  de- 
cay, the  carbon  in  them  unites  with  oxygen  and  forms  carbon 
dioxide. 

The  food  which  we  eat  is  either  animal  or  vegetable,  and 
it  is  made  ready  for  bodily  use  by  a  slow  process  of  burning 
within  the  body ;  carbon  dioxide  accompanies  this  bodily 
burning  of  food  just  as  it  accompanies  the  fires  with  which 
we  are  more  familiar.  The  carbon  dioxide  thus  produced 
within  the  body  escapes  into  the  atmosphere  with  the  breath. 

We  see  that  the  source  of  carbon  dioxide  is  practically  in- 
exhaustible, coming  as  it  does  from  every  stove,  furnace,  and 
candle,  and  further  with  every  breath  of  a  living  organism. 

46.  Danger  of  Carbon  Dioxide.     When  carbon  dioxide  oc- 


54  BURNING   OR   OXIDATION 

curs  in  large  quantities,  it  is  dangerous  to  health,  because  it 
interferes  with  normal  breathing,  lessening  the  escape  of 
waste  matter  through  the  breath  and  preventing  the  access 
to  the  lungs  of  the  oxygen  necessary  for  life.  Carbon  diox- 
ide is  not  poisonous,  but  it  cuts  off  the  supply  of  oxygen,  just 
as  water  cuts  it  off  from  a  drowning  man. 

Since  every  man,  woman,  and  child  constantly  breathes 
forth  carbon  dioxide,  the  danger  in  overcrowded  rooms  is 
great,  and  proper  ventilation  is  of  vital  importance. 

47.  Ventilation.     In  estimating  the  quantity  of  air  neces- 
sary to  keep  a  room  well  aired,  we  must  take  into  account 
the  number  of   lights  (electric   lights  do  not  count)  to  be 
used,  and  the  number  of  people  to  occupy  the  room.     The 
average  house  should  provide  at  the  minimum  600  cubic  feet 
of  space  for  each  person,  and  in  addition,  arrangements  for 
allowing  300  cubic  feet  of  fresh  air  to  enter  every  hour. 

In  houses  which  have  not  a  ventilating  system,  the  air 
should  be  kept  fresh  by  intelligent  action  in  the  opening  of 
doors  and  windows ;  and  since  relatively  few  houses  are 
equipped  with  a  satisfactory  system,  the  following  sugges- 
tions relative  to  intelligent  ventilation  are  offered. 

1.  Avoid  drafts  in  ventilation. 

2.  Ventilate  on  the  sheltered  side  of  the  house.      If  the 
wind  is  blowing  from  the  north,  open  south  windows. 

48.  What  Becomes  of  the  Carbon  Dioxide.    When  we  re- 
flect that  carbon  dioxide  is  constantly  being  supplied  to  the 
atmosphere  and  that  it  is  injurious  to  health,  the  question 
naturally  arises  as  to  how  the  air  remains  free  enough  of  the 
gas  to  support  life.     This  is  largely  because  carbon  dioxide 
is  an  essential  food  of  plants.     Through  their  leaves  plants 
absorb  it  from  the  atmosphere,  and  by  a  wonderful  process 
break  it  up  into  its   component   parts,  oxygen   and   carbon. 
They  reject  the  oxygen,  which  passes  back  to  the  air,  but  they 


A    COMMERCIAL   USE  OF  CARBON  DIOXIDE  55 

retain  the  carbon,  which  becomes  a  part  of  the  plant  structure. 
Plants  thus  serve  to  keep  the  atmosphere  free  from  an  ex- 
cess of  carbon  dioxide  and,  in  addition,  furnish  oxygen  to 
the  atmosphere. 

49.  How  to   Obtain   Carbon   Dioxide.     There   are   several 
ways  in  which    carbon    dioxide    can    be    produced    commer- 
cially,  but  for  laboratory   use  the   simplest   is   to   mix  in   a 
test  tube  powdered  marble,  or  chalk,  and  hydrochloric  acid, 
and  to  collect  the  effervescing  gas  as  shown  in  Figure  24. 
The  substance  which  re- 
mains  in    the    test    tube 

after  the  gas  has  passed 
off  is  a  solution  of  a  salt 
and  water.  From  a  mix- 
ture of  hydrochloric  acid 
(HC1)  and  marble  are  ob- 
tained a  salt,  water,  and 
carbon  dioxide,  the  de- 
sired gas. 

50.  P^.    Commercial    Use    FIG.  24.  —  Making  carbon   dioxide  from   marble 

of  Carbon   Dioxide.    If  a 

lighted  splinter  is  thrust  into  a  test  tube  containing  carbon  di- 
oxide, it  is  promptly  extinguished,  because  carbon  dioxide 
cannot  support  combustion  ;  if  a  stream  of  carbon  dioxide 
and  water  falls  upon  a  fire,  it  atts  like  a  blanket,  covering 
the  flames  and  extinguishing  thern.  The  value  of  a  fire 
extinguisher  depends  upon  the  amount  of  carbon  dioxide  and 
water  which  it  can  furnish.  A  fire  extinguisher  is  a  metal 
case  containing  a  solution  of  bicarbonate  of  soda,  and  a  glass 
vessel  full  of  strong  sulphuric  acid.  As  long  as  the  extin- 
guisher is  in  an  upright  position,  these  substances  are  kept 
separate,  but  when  the  extinguisher  is  inverted,  the  acid  escapes 
from  the  bottle,  and  mixes  with  the  soda  solution.  The  min- 


BURNING   OR   OXIDATION 


gling  liquids  interact  and  liberate  carbon  dioxide.  A  part  of 
the  gas  thus  liberated  dissolves  in  the 
water  of  the  soda  solution  and  escapes 
from  the  tube  with  the  outflowing 
liquid,  while  a  portion  remains  undis- 
solved  and  escapes  as  a  stream  of  gas. 
The  fire  extinguisher  is  therefore  the 
source  of  a  liquid  containing  the  fire- 
extinguishing  substance  and  further 
the  source  of  a  stream  of  carbon  diox- 
ide gas. 

51.  Carbon.  Although  carbon  di- 
oxide is  very  injurious  to  health,  both 
of  the  substances  of  which  it  is  com- 
posed are  necessary  to  life.  We  our- 
selves, our  bones  and  flesh  in  partic- 
ular, are  partly  carbon,  and  every 
animal,  no  matter  how  small  or  insig- 
nificant, contains  some  carbon ;  while 
the  plants  around  us,  the  trees,  the 
grass,  the  flowers,  contain  a  by  no 
means  meager  quantity  of  carbon. 

Carbon  plays  an  important  and  va- 
ried role  in  our  life,  and,  in  some  one  of  its  many  forms, 
enters  into  the  composition  of  most  of  the  substances  which 
are  of  service  and  value  to  man.  The  food  we  eat,  the 
clothes  we  wear,  the  wood  and  coal  we  burn,  the  marble  we 
employ  in  building,  the  indispensable  soap,  and  the  orna- 
mental diamond,  all  contain  carbon  in  some  form. 

52.  Charcoal.  One  of  the  most  valuable  forms  of  carbon 
is  charcoal ;  valuable  not  in  the  sense  that  it  costs  hundreds 
of  dollars,  but  in  the  more  vital  sense,  that  its  use  adds  to 
the  cleanliness,  comfort,  and  health  of  man. 


FlG.  25. — Inside  view  of   a 
fire  extinguisher. 


HOW  CHARCOAL  IS  MADE  57 

The  foul,  bad-smelling  gases  which  arise  from  sewers  can 
be  prevented  from  escaping  and  passing  to  streets  and  build- 
ings by  placing  charcoal  niters  at  the  sewer  exits.  Charcoal 
is  porous  and  absorbs  foul  gases,  and  thus  keeps  the  region 
surrounding  sewers  sweet,  and  clean  and  free  of  odor.  Good 
housekeepers  drop  small  bits  of  charcoal  into  vases  of  flowers 
to  prevent  discoloration  of  the  water  and  the  odor  of  decaying 
stems. 

If  impure  water  niters  through  charcoal,  it  emerges  pure, 
having  left  its  impurities  in  the  pores  of  the  charcoal.  Prac- 
tically all  household  filters  of  drinking  water  are  made  of 
charcoal.  But  such  a  device  may  be  a  source  of  disease  in- 
stead of  a  prevention  of  disease,  unless  the  filter  is  regularly 
cleaned  or  renewed.  This  is  because  the  pores  soon  become 
clogged  with  the  impurities,  and  unless  they  are  cleaned,  the 
water  which  flows  through  the  filter  passes  through  a  bed  of 
impurities  and  becomes  contaminated  rather  than  purified. 
Frequent  cleansing  or  renewal  of  the  filter  removes  this  diffi- 
culty. 

Commercially,  charcoal  is  used  on  a  large  scale  in  the  refin- 
ing of  sugars,  sirups,  and  oils.  Sugar,  whether  it  comes 
from  the  maple  tree,  or  the  sugar  cane,  or  the  beet,  is  dark 
colored.  It  is  whitened  by  passage  through  filters  of  finely 
pulverized  charcoal.  Cider  and  vinegar  are  likewise  cleared 
by  passage  through  charcoal. 

The  value  of  carbon,  in  the  form  of  charcoal,  as  a  purifier 
is  very  great,  whether  we  consider  it  a  deodorizer,  as  in  the 
case  of  the  sewage,  or  a  decolorizer,  as  in  the  case  of  the  re- 
fineries, or  whether  we  consider  the  service  it  has  rendered 
man  in  the  elimination  of  danger  from  drinking  water. 

53.  How  Charcoal  is  Made.  Charcoal  may  be  made  by 
heating  wood  in  an  oven  to  which  air  does  not  have  free 
access.  The  absence  of  air  prevents  ordinary  combustion, 


58  BURNING   OR   OXIDATION 

nevertheless  the  intense  heat  affects  the  wood  and  changes 
it  into  new  substances,  one  of  which  is  charcoal. 

The  wood  which  smolders  on  the  hearth  and  in  the  stove 
is  charcoal  in  the  making.  Formerly  wood  was  piled  in 
heaps,  covered  with  sod  or  sand  to  prevent  access  of  oxygen, 
and  then  was  set  fire  to;  the  smoldering  wood,  cut  off  from  an 
adequate  supply  of  air,  was  slowly  transformed  into  charcoal. 
Scattered  over  the  country  one  still  finds  isolated  charcoal 
kilns,  crude  earthen  receptacles,  in  which  wood  thus  deprived 
of  air  was  allowed  to  smolder  and  form  charcoal.  A  student 
can  make  in  the  laboratory  sufficient  charcoal  for  art  lessons 
by  heating  in  an  earthen  vessel  wood  buried  in  sand.  The 
process  will  be  slow,  however,  because  the  heat  furnished  by 
a  Bunsen  burner  is  not  great,  and  the  wood  is  transformed 
slowly. 

A  form  of  charcoal  known  as  animal  charcoal,  or  bone 
black,  is  obtained  from  the  charred  remains  of  animals  rather 
than  plants,  and  may  be  prepared  by  burning  bones  and 
animal  refuse  as  in  the  case  of  the  wood. 

54.  Matter  and  Energy.  When  wood  is  burned,  a  small 
pile  of  ashes  is  left,  and  we  think  of  the  bulk  of  the  wood  as 
destroyed.  It  is  true  we  have  less  matter  that  is  available  for 
use  or  that  is  visible  to  sight,  but,  nevertheless,  no  matter  has 
been  destroyed.  The  matter  of  which  the  wood  is  composed 
has  merely  changed  its  character ,  some  of  it  is  in  the  condition 
of  ashes,  and  some  in  the  condition  of  invisible  gases,  such  as 
carbon  dioxide,  but  none  of  it  has  been  destroyed.  It  is  a 
principle  of  science  that  matter  can  neither  be  destroyed  nor 
created ;  it  can  only  be  changed,  or  transformed,  and  it  is  our 
business  to  see  that  we  do  not  heedlessly  transform  it  into 
substances  which  are  valueless  to  us  and  our  descendants; 
as,  for  example,  when  our  magnificent  forests  are  recklessly 
wasted.  The  smoke,  gases,  and  ashes  left  in  the  path  of  a 


MATTER  AND  ENERGY  59 

raging  forest  fire  are  no  compensation  to  us  for  the  valuable 
timber  destroyed.  The  sum  total  of  matter  has  not  been 
changed,  but  the  amount  of  matter  which  man  can  use  has 
been  greatly  lessened. 

The  principle  just  stated  embodies  one  of  the  fundamental 
laws  of  science,  called  the  law  of  the  conservation  of  matter. 

A  similar  law  holds  for  energy  as  well.  We  can  transform 
electric  energy  into  the  motion  of  trolley  cars,  or  we  can  make 
use  of  the  energy  of  streams  to  turn  the  wheels  of  our  mills, 
but  in  all  these  cases  we  are  transforming,  not  creating, 
energy. 

When  a  ball  is  fired  from  a  rifle,  most  of  the  energy  of  the 
gunpowder  is  utilized  in  motion,  but  some  is  dissipated  in  pro- 
ducing a  flash  and  a  report,  and  in  heat.  The  energy  of  the 
gunpowder  has  been  scattered,  but  the  sum  of  the  various 
forms  of  energy  is  equal  to  the  energy  originally  stored  away 
in  the  powder.  The  better  the  gun  is,  the  less  will  be  the 
energy  dissipated  in  smoke  and  heat  and  noise. 


CHAPTER   V 

FOOD 

55.  The  Body  as  a  Machine.     Wholesome  food  and  fresh 
air   are  necessary  for  a  healthy  body.     Many  housewives, 
through  ignorance,  supply   to  their  hard-working  husbands 
and  their  growing  sons  and  daughters  food  which  satisfies 
the  appetite,  but  which  does  not  give  to  the  body  the  elements 
needed  for  daily   work  and   growth.     Some  foods,  such  as 
lettuce,  cucumbers,  and  watermelons,  make  proper  and  satis- 
factory changes  in  diet,  but  are  not  strength  giving.     Other 
foods,  like  peas  and  beans,  not    only    satisfy  the    appetite, 
but  supply  to   the  body  abundant  nourishment.     Many  im- 
migrants  live   cheaply  and  well  with   beans   and   bread   as 
their  main  diet. 

It  is  of  vital  importance  that  the  relative  value  of  different 
foods  as  heat  producers  be  known  definitely ;  and  jftst  as  the 
yard  measures  length  and  the  pound  measures  weight  the 
calorie  is  used  to  measure  the  amount  of  heat  which  a  food  is 
capable  of  furnishing  to  the  body.  Our  bodies  are  human 
machines,  and,  like  all  other  machines,  require  fuel  for  their 
maintenance.  The  fuel  supplied  to  an  engine  is  not  all  avail- 
able for  pulling  the  cars ;  a  large  portion  of  the  fuel  is  lost  in 
smoke,  and  another  portion  is  wasted  as  ashes.  So  it  is  with 
the  fuel  that  runs  the  body.  The  food  we  eat  is  not  all  avail- 
able for  nourishment,  much  of  it  being  as  useless  to  us  as  are 
smoke  and  ashes  to  an  engine.  The  best  foods  are  those 
which  do  the  most  for  us  with  the  least  possible  waste. 

56.  Fuel  Value.     By  fuel  value  is  meant  the  capacity  foods 
have  for   yielding   heat   to   the   body.     The    fuel   value   of 
the  foods  we  eat  daily  is  so  important  a  factor  in  life  that 

60 


FUEL    VALUE 


6l 


physicians,  dietitians,  nurses,  and  those  having  the  care  of 
institutional  cooking  acquaint  themselves  with  the  relative 
fuel  values  of  practically  all  of  the  important  food  substances. 
The  life  or  death  of  a  patient  may  be  determined  by  the 
patient's  diet,  and  the  working  and  earning  capacity  of  a 
father  depends  largely  upon  his  prosaic  three  meals.  An 
ounce  of  fat,  whether  it  is  the  fat  of  meat  or  the  fat  of  olive 
oil  or  the  fat  of  any  other  food,  produces  in  the  body  two  and 
a  quarter  times  as  much  heat  as  an  ounce  of  starch.  Of  the 
vegetables,  beans  provide  the  greatest  nourishment  at  the 
least  cost,  and  to  a  large  extent  may  be  substituted 
for  meat.  It  is  not  uncommon  to  find  an  outdoor 
laborer  consuming  one  pound  of  beans  per  day, 
and  taking  meat  only  on  "high  days  and  holidays." 
The  fuel  value  of  a  food  is  determined  by  means 
of  the  bomb  calorimeter  (Fig.  26).  The  food  sub- 
stance is  put  into  a  cham- 
ber A  and  ignited,  and  the 
heat  of  the  burning  sub- 
stance raises  the  temper- 
ature of  the  water  in  the  sur- 
rounding vessel.  If  1000 
grams  of  water  are  in  the 
vessel,  and  the  temperature 
of  the  water  is  raised  2°  C, 
the  number  of  calories  pro- 
duced by  the  substance 
would  be  2000,  and  the  fuel 
value  would  be  2000  calories.*  From  this  the  fuel  value 
of  one  quart  or  one  pound  of  the  substance  can  be  deter- 

*  As  applied  to  food,  the  calorie  is  greater  than  that  used  in  the  ordinary 
laboratory  work,  being  the  amount  of  heat  necessary  to  raise  the  temperature  of 
1000  grams  of  water  i°C,  rather  than  I  gram  I°C. 


FIG.  26.  —  The  bomb  calorimeter  from  which 
the  fuel  value  of  food  can  be  estimated. 


62  FOOD 

mined,  and  the  food  substance  will  be  said  to  furnish 
the  body  with  that  number  of  heat  units,  providing  all  of  the 
pound  of  food  were  properly  digested. 

Leg  of  lean  mutton  furnishes 790  calories  per  pound 

Rib  of  beef  furnishes 1150  calories  per  pound 

Shad  furnishes 380  calories  per  pound 

Chicken  furnishes 505  calories  per  pound 

Apples  furnish 290  calories  per  pound 

Bananas  furnish 460  calories  per  pound 

Prunes  furnish 370  calories  per  pound 

Watermelons  furnish 140  calories  per  pound 

Lima  beans  furnish 570  calories  per  pound 

Beets  furnish 215  calories  per  pound 

Carrots  furnish 210  calories  per  pound 

Lettuce  furnishes  .          90  calories  per  pound 

Onion  furnishes 225  calories  per  pound 

Cucumber  furnishes 80  calories  per  pound 

Almonds  furnish 3030  calories  per  pound 

Butternuts  furnish 316$  calories  per  pound 

Walnuts  furnish    .     .::.•". 3306  calories  per  pound 

Peanuts  furnish 2560  calories  .per  pound 

Brazil  nuts  furnish 3265  calories  per  pound 

Oatmeal  furnishes 4673  calories  per  pound 

Rolled  wheat  furnishes 4175  calories  per  pound 

Macaroni  furnishes 1665  calories  per  pound 

57.  Varied  Diet.  The  human  body  is  a  much  more  varied 
and  complex  machine  than  any  ever  devised  by  man  ;  personal 
peculiarities,  as  well  as  fuel  values,  influence  very  largely  the 
diet  of  an  individual.  Strawberries  are  excluded  from  some 
diets  because  of  a  rash  which  is  produced  on  the  skin,  pork 
is  excluded  from  other  diets  for  a  like  reason  ;  cauliflower 
is  absolutely  indigestible  to  some  and  is  readily  digested  by 
others.  From  practically  every  diet  some  foods  must  be 
excluded,  no  matter  what  the  fuel  value  of  the  substance  maybe. 

Then,  too,  there  are  more  uses  for  food  than  the  production 
of  heat.  Teeth  and  bones  and  nails  need  a  constant  supply 


WHY   WE  EAT  SO  MUCH  63 

of  mineral  matter,  and  mineral  matter  is  frequently  found  in 
greatest  abundance  in  foods  of  low  fuel  value,  such  as  lettuce, 
watercress,  etc.,  though  practically  all  foods  yield  at  least  a 
small  mineral  constituent.  When  fuel  values  alone  are  con- 
sidered, fruits  have  a  low  value,  but  because  of  the  flavor  they 
impart  to  other  foods,  and  because  of  the  healthful  influence 
they  exercise  in  digestion,  they  cannot  be  excluded  from  the 
diet. 

Care  should  be  constantly  exercised  to  provide  substantial 
foods  of  high  fuel  value.  But  the  nutritive  foods  should  be 
wisely  supplemented  by  such  foods  as  fruits,  whose  real  value 
is  one  of  indirect  rather  then  direct  service. 

58.  Our  Bodies.     Somewhat  as  a  house  is  composed  of  a 
group  of  bricks,  or  a  sand  heap  of  grains  of  sand,  the  human 
body  is  composed  of  small  divisions  called  cells.     Ordinarily 
we  cannot  see  these  cells  because  of  their  minuteness,  but  if 
we  examine  a  piece  of  skin,  or  a  hair  of  the  head,  or  a  tiny 
sliver  of  bone  under  the  microscope,  we  see  that  each  of  these 
is  composed  of  a  group  of  different  cells.     A  merchant,  watch- 
ful about  the  fineness  of  the  wool  which  he  is  purchasing, 
counts  with  his  lens  the  number  of  threads  to  the  inch ;   a 
physician,  when  he  wishes,  can,  with  the  aid  of  the  micro- 
scope, examine  the  cells  in  a  muscle,  or  in  a  piece  of  fat,  or  in 
a  nerve  fiber.     Not  only  is  the  human  body  composed  of 
cells,  but  so  also  are  the  bodies  of  all  animals  from  the  tiny 
gnat  which  annoys  us,  and  the  fly  which  buzzes  around  us, 
to  the  mammoth  creatures  of  the  tropics.     These  cells  do  the 
work  of  the  body,  the  bone  cells  build  up  the  skeleton,  the 
nail  cells  form  the  finger  and  toe  nails,  the  lung  cells  take' 
care  of  breathing,  the  muscle  cells  control  motion,  and  the 
brain  cells  are  responsible  for  thought. 

59.  Why  we   eat   so   Much.     The  cells   of  the  body  are 
constantly,  day  by  day,  minute  by  minute,  breaking  down 


64  FOOD 

and  needing  repair,  are  constantly  requiring  replacement  by 
new  cells,  and,  in  the  case  of  the  child,  are  continually  in- 
creasing in  number.  The  repair  of  an  ordinary  machine, 
an  engine,  for  example,  is  made  at  the  expense  of  money, 
but  the  repair  and  replacement  of  our  human  cell  machinery 
are  accomplished  at  the  expense  of  food.  More  than  one 
third  of  all  the  food  we  eat  goes  to  maintain  the  body  cells, 
and  to  keep  them  in  good  order.  It  is  for  this  reason  that 
we  consume  a  large  quantity  of  food.  If  all  the  food  we  eat 
were  utilized  for  energy,  the  housewife  could  cook  less,  and 
the  housefather  could  save  money  on  grocer's  and  butcher's 
bills.  If  you  put  a  ton  of  coal  in  an  engine,  its  available 
energy  is  used  to  run  the  engine,  but  if  the  engine  were  like 
the  human  body,  one  third  of  the  ton  would  be  used  up  by 
the  engine  in  keeping  walls,  shafts,  wheels,  belts,  etc.,  in 
order,  and  only  two  thirds  would  go  towards  running  the 
engine.  When  an  engine  is  not  working,  fuel  is  not  con- 
sumed, but  the  body  requires  food  for  mere  existence,  regard- 
less of  whether  it  does  active  work  or  not.  When  we  work, 
the  cells  break  down  more  quickly,  and  the  repair  is  greater 
than  when  we  are  at  rest,  and  hence  there  is  need  of  a  larger 
amount  of  food ;  but  whether  we  work  or  not,  food  is  necessary. 

60.  The  Different  Foods.     The  body  is  very  exacting  in  its 
demands,  requiring  certain  definite  foods  for  the  formation 
and  maintenance  of  its  cells,  and  other  foods,  equally  definite, 
but  of  different  character,  for  heat;    our  diet  therefore  must 
contain  foods  of  high  fuel  value,  and  likewise  foods  of  cell- 
forming  power. 

Although  the  foods  which  we  eat  are  of  widely  different 
character,  such  as  fruits,  vegetables,  cereals,  oils,  meats,  eggs, 
milk,  cheese,  etc.,  they  can  be  put  into  three  great  classes : 
the  carbohydrates,  the  fats,  and  the  proteids. 

61.  The   Carbohydrates.     Corn,    wheat,    rye,    in    fact   all 


THE  FATS  65 

cereals  and  grains,  potatoes,  and  most  vegetables  are  rich  in 
carbohydrates ;  as  are  also  sugar,  molasses,  honey,  and  maple 
sirup.  The  foods  of  the  first  group  are  valuable  because  of 
the  starch  they  contain ;  for  example,  corn  starch,  wheat 
starch,  potato  starch.  The  substances  of  the  second  group 
are  valuable  because  of  the  sugar  they  contain  ;  sugar  contains 
the  maximum  amount  of  carbohydrate.  In  the  sirups  there 
is  a  considerable  quantity  of  sugar,  while  in  some  fruits  it  is 
present  in  more  or  less  dilute  form.  Sweet  peaches,  apples, 
grapes,  contain  a  moderate  amount  of  sugar;  watermelons, 
pears,  etc.,  contain  less. 

Carbohydrates,  whether  of  the  starch  group  or  the  sugar 
group,  are  composed  chiefly  of  three  elements :  carbon, 
hydrogen,  and  oxygen  ;  they  are  therefore  combustible,  and 
are  great  energy  producers.  On  the  other  hand,  they  are 
worthless  for  cell  growth  and  repair,  and  if  we  limited  our 
diet  to  carbohydrates,  we  should  be  like  a  man  who  had  fuel 
but  no  engine  capable  of  using  it. 

62.  The  Fats.  The  best-known  fats  are  butter,  lard,  olive 
oil,  and  the  fats  of  meats,  cheese,  and  chocolate.  When  we 
test  fats  for  fuel  values  by  means  of  a  calorimeter  (Fig.  26),  we 
find  that  they  yield  twice  as  much  heat  as  the  carbohydrates, 
but  that  they  burn  out  more  quickly.  Dwellers  in  cold  climates 
must  constantly  eat  large  quantities  of  fatty  foods  if  they  are 
to  keep  their  bodies  warm  and  survive  the  extreme  cold. 
Cod  liver  oil  is  an  excellent  food  medicine,  and  if  taken  in 
winter  serves  to  warm  the  body  and  to  protect  it  against  the 
rigors  of  cold  weather.  The  average  person  avoids  fatty 
foods  in  summer,  knowing  from  experience  that  rich  foods 
make  him  warm  and  uncomfortable.  The  harder  we  work  and 
the  colder  the  weather,  the  more  food  of  that  kind  do  we  re- 
quire ;  it  is  said  that  a  lumberman  doing  heavy  out-of-door  work 
in  cold  climates  needs  three  times  as  much  food  as  a  city  clerk. 

CL.    GEN.    SCI.  —  5 


66  FOOD 

63.  The   Proteids.     The  proteids  are   the  building  foods, 
furnishing  muscle,  bone,  skin  cells,  etc.,  and  supplying  blood 

and  other  bodily  fluids.  The 
best-known  proteids  are  white 
of  egg,  and  lean  of  fish  and 
meat ;  peas  and  beans  have 
~r^  also  an  abundant  supply  of  this 

FIG.  27.  _«  is  the  amount  of  fat  neces-  Substance.  This  class  of  foods 
sary  to  make  one  calorie;  b  is  the  contains  Carbon,  Oxygen,  and 
amount  of  sugar  or  proteid  necessary  to  .  ,  -  .  -,-,.. 

make  one  calorie.  hydrogen,  and  in  addition,  two 

substances  not  found  in  carbo- 
hydrates or  fats  —  namely,  nitrogen  and  sulphur.  Since  the 
proteids  contain  all  the  elements  found  in  the  two  other 
classes  of  foods,  they  are  able  to  contribute,  if  necessary,  to 
the  store  of  bodily  energy ;  but  their  main  function  is  up- 
building, and  the  diet  should  be  chosen  so  that  the  pro- 
teids do  not  have  a  double  task. 

For  an  average  man  four  ounces  of  dry  proteid  matter 
daily  will  suffice  to  keep  the  body  cells  in  normal  condition. 

It  has  been  estimated  that  300,000,000  blood  cells  alone 
need  daily  repair  or  renewal.  When  we  consider  that  the 
blood  is  but  one  part  of  the  body,  and  that  all  organs  and 
fluids  have  corresponding  requirements,  we  realize  how  vast 
is  the  work  to  be  done  by  the  food  which  we  eat. 

64.  Mistakes  in  Buying.     The  body  demands  a  daily  ration 
of  the  three  classes  of  food  stuffs,  but  it  is  for  us  to  determine 
from  what  meats,  vegetables,  fruits,  cereals,  etc.,  this  supply 
shall  be  obtained  (Figs.  28  and  29). 

Generally  speaking,  meats  are  the  most  expensive  foods  we 
can  purchase,  and  hence  should  be  bought  seldom  and  in 
small  quantities.  Their  place  can  be  taken  by  beans,  peas, 
potatoes,  etc.,  and  at  less  than  a  quarter  of  the  cost.  The 
average  American  family  eats  meat  three  times  a  day,  while 


MISTAKES  IN  BUYING 


67 


the    average   faniily   of   the    more    conservative    and    older 
countries  rarely  eats  meat  more  than  once  a  day.     The  fol- 


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68 


FOOD 


&  meal 
2lbs. 
^Scents 


FIG.  29.  —  Diagram  showing  the  difference  in  the  cost  of  three  foods  which  give  about 
the  same  amount  of  nutrition  each. 


lowing  tables  indicate  the  financial  loss  arising  from  an  unwise 
selection  of  foods  :  - 

FOOD  CONSUMED  —  ONE  WEEK 


FAMILY  No.  i 


FAMILY  No.  2 


20  loaves  of  bread  .     .     .     .  $  i  .00 
10  to  12  Ib.  loin  steak  or  meat 

of  similar  cost      ....     2.00 
20  to  25  Ib.  rib  roast  or  simi- 
lar meat 4.40 

4    Ib.      high-priced      cereal 

breakfast  food,  20^  .  .  .  .80 
Cake  and  pastry  purchased  .  3.00 
8  Ib.  butter,  30 f-  .  .  .  .  .  2.40 
Tea,  coffee,  spices,  etc.  .  .  .75 
Mushrooms 75 


15  Ib.  flour,  bread  home-made 

(skim  milk  used)      ...     $  .45 

Yeast,  shortening,  and  skim 

milk x.        .10 

10  Ib.    steak    (round,   Ham- 
burger, and  some  loin)       .      1.50 

10  Ib.  other  meats,   boiling 
pieces,  rump  roast,  etc.       .      i.oo 

5  Ib.  cheese,  16^     .     .     .     .        .80 

5  Ib.  oatmeal  (bulk)     ...        .15 

5  Ib.  beans 25 


MISTAKES  IN  BUYING 


69 


FAMILY  No.  i 


FAMILY  No.  2 


Celery i.oo 

Oranges 2.00 

Potatoes 25 

Miscellaneous  canned  goods       2.00 

Milk 50 

Miscellaneous  foods     .     .     .      2.00 
3  doz.  eggs -60 


$23.45 


Home-made  cake  and  pastry 
6  Ib.  butter,  30  f  .... 
3  Ib.  home-made  shortening 
Tea,  coffee,  and  spices  .  . 

Apples       . 

Prunes 

Potatoes 

Milk 

Miscellaneous  foods 

3  doz.  eggs 


i.oo 
i. 80 
.25 
.40 
.50 
.25 
.25 

.1.00 
I.OO 

.60 


$11.30 


"  The  tables  show  that  one  family  spends  over  twice  as 
much  in  the  purchase  of  foods  as  the  other  family,  and  yet 
the  one  whose  food  costs  the  less  actually  secures  the  larger 
amount  of  nutritive  material  and  is  better  fed  than  the  family 
where  more  money  is  expended."  —  From  Human  Foods, 
Snyder. 


CHAPTER   VI 


WATER 

65.  Destructive  Action  of  Water.  The  action  of  water  in 
stream  and  sea,  in  springs  and  wells,  is  evident  to  all ;  but 
the  activity  of  ground  water  —  that  is,  rain  water  which  sinks 
into  the  soil  and  remains  there — is  little  known  in  general- 
The  real  activity  of  ground  water  is  due  to  its  great  solvent 
power  ;  every  time  we  put  sugar  into  tea  or  soap  into  water  we 
are  using  water  as  a  solvent.  When  rain  falls,  it  dissolves  sub- 
stances floating  in  the  atmosphere,  and  when  it  sinks  into  the 
ground  and  becomes  ground  water,  it  dissolves  material  out 
of  the  rock  which  it  encounters  (Fig.  30).  We  know  that  water 
contains  some  mineral  matter,  because  kettles  in  which  water 
is  boiled  acquire  in  a  short  time  a  crust  or  coating  on  the  in- 
side. This  crust  is  due  to  the  accumulation  in  the  kettle  of 

mineral  matter  which 
was  in  solution  in  the 
water,  but  which  was 
left  behind  when  the 
water  evaporated.  (See 
Section  25.) 

The  amount  of  dis- 
solved mineral  matter 
present  in  some  wells 
and  springs  is  surprisingly  great ;  the  famous  springs  of  Bath, 
England,  contain  so  much  mineral  matter  in  solution,  that  a 
column  9  feet  in  diameter  and  140  feet  high  could  be  built 

70 


FlG.  30.  —  Showing  how  coves  and  holes  are 
formed  by  the  solvent  action  of  water. 


DESTRUCTIVE  ACTION  OF   WATER 


out  of  the  mineral  matter  contained  in   the  water  consumed 
yearly  by  the  townspeople. 

Rocks  and  minerals  are  not  all  equally  soluble  in  water ; 
some  are  so  little  soluble  that  it  is  years  before  any  change 
becomes  apparent,  and  the  substances  are  said  to  be  insolu- 
ble, yet  in  reality  they  are  slowly  dissolving.  Other  rocks, 
like  limestone,  are  so  readily  soluble  in  water  that  from  the 
small  pores  and  cavities  eaten  out  by  the  water,  there  may 
develop  in  long  centuries,  caves  and  caverns  (Fig.  30).  Most 
rock,  like  granite,  con- 
tains several  sub- 
stances, some  of  which 
are  readily  soluble  and 
others  of  which  are  not 
readily  soluble;  in  such 
rocks  a  peculiar  ap- 
pearance is  presented, 
due  to  the  rapid  disap- 
pearance of  the  soluble 
substance,  and  the  per- 
sistence of  the  more  re- 
sistant substance  (Fig. 

SO- 

We  see  that  the 
solvent  power  of  water 
is  constantly  causing 
changes,  dissolving 
some  mineral  substan- 
ces, and  leaving  others 
practically  untouched ; 
eating  out  crevices  of  various  shapes  and  sizes,  and  by  gradual 
solution  through  unnumbered  years  enlarging  these  crevices 
into  wonderful  caves,  such  as  the  Mammoth  Cave  of  Kentucky. 


FIG.  31.  —  The  work  of  water  as  a  solvent. 


WATER 


FIG.  32.  —  From  the  mingling  of  two  liquids  a 
solid  is  sometimes  formed. 


66.  Constructive  Action  of  Water.  Water  does  not  always 
act  as  a  destructive  agent ;  what  it  breaks  down  in  one  place 
it  builds  up  in  another.  It  does  this  by  means  of  precipita- 
tion. Water  dissolves  salt,  and  also  dissolves  lead  nitrate,  but 
if  a  salt  solution  is  mixed  with  a  lead  nitrate  solution,  a  solid 
white  substance  is  formed  in  the  water  (Fig.  32).  This  for- 
mation of  a  solid  substance 
from  the  mingling  of  two 
liquids  is  called  precipita- 
tion ;  such  a  process  occurs 
daily  in  the  rocks  beneath 
the  surface  of  the  earth. 
(See  Laboratory  Manual.) 
Suppose  water  from  dif- 
ferent sources  enters  a 
crack  in  a  rock,  bringing 
different  substances  in  so- 
lution ;  then  the  mingling  of  the  waters  may  cause  precipita- 
tion, and  the  solid  thus  formed  will  be  deposited  in  the  crack 
and  fill  it  up.  Hence,  while  ground  water  tends  to  make  rock 
porous  and  weak  by  dissolving  out  of  it  large  quantities  of 
mineral  matter,  it  also  tends  under  other  conditions.,  to  rriake 
it  more  compact  because  it  deposits  in  cracks,  crevices,  and 
pores  the  mineral  matter- precipitated  from  solution. 

These  two  forces  are  constantly  at  work ;  in  some  placed  the 
destructive  action  is  mere  prominent,  in  other  places  the  con- 
structive action ;  but  always  the  result  is  to  change  the  charac- 
ter of  the  original  substance.  When  the  mineral  matter  pre- 
cipitated from  the  solutions  is  deposited  in  cracks,  veins  are 
formed  (Fig.  33),  which  may  consist  of  the  ore  of  different 
metals,  such  as  gold,  silver,  copper,  lead,  etc.  Man  is  almost 
entirely  dependent  upon  these  veins  for  the  supply  of  metal 
needed  in  the  various  industries,  because  in  the  original  con- 


STREAMS 


73 


dition  of  the  rocks,  the  metallic  substances  are  so  scattered 
that  they  cannot  be  profitably  extracted. 

Naturally,  the  veins  themselves  are  not  composed  of  one 
substance  alone,  because  several  different  precipitates  may 
be  formed.  But  there  is  a 
decided  grouping  of  valu- 
able metals,  and  these  can 
then  be  readily  separated 
by  means  of  electricity. 

67.  Streams.  Streams 
usually  carry  mud  and  sand 
along  with  them ;  this  is 
particularly  well  seen  after 
a  storm  when  rivers  and 
brooks  are  muddy.  The 
puddles  which  collect  at 
the  foot  of  a  hill  after  a 
storm  are  muddy  because 
of  the  particles  of  soil 
gathered  by  the  water  as 
it  runs  down  the  hill.  The 
particles-  are  not  dissolved 
in  the  water,  but  are  held 
there  in  suspension,  as  we 
call  it  technically.  The  river  made  muddy  after  a  storm  by 
suspended  particles  usually  becomes  clear  and  transparent 
after  it  has  traveled  onward  for  miles,  because,  as  it  travels, 
the  particles  drop  to  the  bottom  and  aw*  deposited  there. 
Hence,  materials  suspended  in  the  water  are  borne  along 
and  deposited  at  various  places  (Fig.  34).  The  amount  of 
deposition  by  large  rivers  is  so  great  that  in  some  places 
channels  fill  up  and  must  be  dredged  annually,  and  vessels  are 
sometimes  caught  in  the  deposit  and  have  to  be  towed  away. 


FIG.  33.  —  Mineral  matter  precipitated  fron 
solution  is  deposited  in  crevices  and  forms 
veins. 


74 


WATER 


Running  water  in  the  form  of  streams  and  rivers,  by  carry- 
ing sand  particles,  stones,  and  rocks  from  high  slopes  and 


FIG.  34.  —  Deposit  left  by  running  water. 

depositing  them  at  lower  levels,  wears  away  land  at  one  place 
and  builds  it  up  at  another,  and  never  ceases  in  its  work  of 
changing  the  nature  of  the  earth's  surface  (Fig.  35). 


FIG.  35.  —  Water  by  its  action  constantly  changes  the  character  of  the  land. 

68.   Relation  of  Water  to  Human  Life.     Water  is  one  of  the 
most  essential  of  food"  materials,  and  whether  we  drink  much 


WATER   AND  ITS  DANGERS 


75 


or  little  water,  we  nevertheless  get  a  great  deal  of  it.  The 
larger  part  of  many  of  our  foods  is  composed  of  water ;  more 
than  half  of  the  weight  of  the 
meat  we  eat  is  made  up  of 
water  ;  and  vegetables  are  often 
more  than  nine  tenths  water. 
(See  Laboratory  Manual.)  As- 
paragus and  tomatoes  have  over 
90  per  cent  of  water,  and  most 
fruits  are  more  than  three  fourths 
water ;  even  bread,  which  con- 
tains as  little  water  as  any  of  our 
common  foods,  is  about  one  third 
water  (Fig.  36). 

Without  water,  solid  food 
material,  although  present  in  the 
body,  would  not  be  in  a  condi- 
tion suitable  for  bodily  use.  An 
abundant  supply  of  water  enables  the  food  to  be  dissolved  or 
suspended  in  it,  and  in  solution  the  food  material  is  easily 
distributed  to  all  parts  of  the  body. 

Further,  water  assists  in  the  removal  of  the  daily  bodily 
wastes,  and  thus  rids  the  system  of  foul  and  poisonous 
substances. 

The  human  body  itself  consists  largely  of  water ;  indeed, 
about  two  thirds  of  our  own  weight  is  water.  The  constant 
replenishing  of  this  large  quantity  is  necessary  to  life,  and  a 
considerable  amount  of  the  necessary  supply  is  furnished  by 
foods,  particularly  the  fruits  and  vegetables. 

But  while  the  supply  furnished  by  the  daily  food  is 
considerable,  it  is  by  no  means  sufficient,  and  should  be 
supplemented  by  good  drinking  water. 

69.    Water  and  its  Dangers.     Our  drinking  water  comes 


FIG.  36.  —  Diagram  of  the  composition 
of  a  loaf  of  bread  and  of  a  potato  :  i, 
ash ;  2,  food ;  3,  water. 


76  WATER 

from  far  and  near,  and  as  it  moves  from  place  to  place,  it 
carries  with  it  in  solution  or  suspension  anything  which  it 
can  find,  whether  it  be  animal,  vegetable,  or  mineral  matter. 
The  power  of  water  to  gather  up  matter  is  so  great  that 
the  average  drinking  water  contains  20  to  90  grains  of  solid 
matter  per  gallon;  that  is,  if  a  gallon  of  ordinary  drinking 
water  is  left  to  evaporate,  a  residue  of  20  to  90  grains  will  be 
left.  (See  Laboratory  Manual.)  As  water  runs  down  a  hill 


FIG.  37.  —  As  water  flows  over  the  land,  it  gathers  filth  and  disease  germs. 

slope  (Fig.  37),  it  carries  with  it  the  filth  gathered  from  acres 
of  land;  carries  with  it  the  refuse  of  stable,  barn,  and  kitchen ; 
and  too  often  this  impure  surface  water  joins  the  streams  which 
supply  our  cities.  Lakes  and  rivers  which  furnish  drinking 
water  should  be  carefully  protected  from  surface  draining; 
that  is,  from  water  which  has  flowed  over  the  land  and  has 


METHODS  OF  PURIFICATION  77 

thus  accumulated  the  waste  of  pasture  and  stable  and,  it  may 
be,  of  dumping  ground. 

It  is  not  necessary  that  water  should  be  absolutely  free 
from  all  foreign  substances  in  order  to  be  safe  for  daily  use 
in  drinking;  a  limited  amount  of  mineral  matter  »is  not 
injurious  and  may  sometimes  be  really  beneficial.  It  is  the 
presence  of  animal  'and  vegetable  matter  that  causes  real 
danger,  and  it  is  known  that  typhoid  fever  is  due  largely  to 
such  impurities  present  in  the  drinking  water. 

70.  Methods  of  Purification.  Water  is  improved  by  any 
of  the  following  methods:  — 

(a)  Boiling.  The  heat  of  boiling  destroys  animal  and 
vegetable  germs.  Hence  water  that  has  been  boiled  a  few 
minutes  is  safe  to  use.  This  is  the  most  practical  method  of 
purification  in  the  home,  and  is  very  efficient.  The  boiled 
water  should  be  kept  in  clean,  corked  bottles ;  otherwise  for- 
eign substances  from  the  atmosphere  reenter  the  water,  and 
the  advantage  gained  from  boiling  is  lost. 

(b}  Distillation.  By  this  method  pure  water  is  obtained, 
but  this  method  of  purification  cannot  be  used  conveniently  in 
the  home  (Section  25). 

(c)  Filtration.  In  filtration,  the  water  is  forced  through 
porcelain  or  other  porous  substances  which  allow  the  passage 
of  water,  but  which  hold  ba^ck  the  minute  foreign  particles 
suspended  in  the  water.  (See  Laboratory  Manual.)  The  fil- 
ters used  in  ordinary  dwellings  are  of  stone,  asbestos,  or  char- 
coal. They  are  often  valueless,  because  they  soon  become 
choked  and  cannot  be  properly  cleaned. 

The  filtration  plants  owned  and  operated  by  large  cities 
are  usually  safe;  there  is  careful  supervision  of  the  filters, 
and  frequent  and  effective  cleanings  are  made.  In  many 
cities  the  filtration  system  is  so  good  that  private  care  of  the 
water  supply  is  unnecessary. 


WATER 


71.  The  Source  of  Water.  In  the  beginning,  the  earth 
was  stored  with  water  just  as  it  was  with  metal,  rock, 
etc.  Some  of  the  water  gradually  took  the  form  of  rivers, 
lakes,  streams,  and  wells,  as  now,  and  it  is  this  original  supply 
of  water  which  furnishes  us  all  that  we  have  to-day.  We 
quarry  to  obtain  stone  and  marble  for  building,  and  we 
fashion  the  earth's  treasures  into  forms  of  our  own,  but  we 
cannot  create  these  things.  We  bore  into  the  ground  and 
drill  wells  in  order  to  obtain  water  from  hidden  sources; 
we  utilize  rapidly  flowing  streams  to  drive  the  wheels  of 
commerce,  but  the  total  amount  of  water  remains  practically 
unchanged. 

The  water  which  flows  on  the  earth  is  constantly  changing 
its  form ;  the  heat  of  the  sun  causes  it  to  evaporate,  or  to 
become  vapor,  and  to  mingle  with  the  atmosphere.  In  time, 
the  vapor  cools,  condenses,  and  falls  as  snow  or  rain ;  the 
water  which  is  thus  returned  to  the  earth  feeds  our  rivers, 

lakes,  springs,  and 
wells,  and  these  in 
turn  supply  water  to 
man.  When  water 
falls  upon  a  field,  it 
soaks  into  the  ground, 
or  collects  in  puddles 
'  which  slowly  evapo- 

fee 


rate,  or  it  runs  off 
and  drains  into  small 
streams  or  into  rivers. 
That  which  soaks 
into  the  ground  is 

the  most  valuable  because  it  remains  on  the  earth  longest 

and  is  the  purest. 

Water  which  soaks  into  the  ground  moves   slowly  down- 


FlG.  38.  —  How  springs  are  formed.     A,  porous  layer 
B,  non-porous  layer ;   C,  spring. 


THE   COMPOSITION  OF   WATER 


79 


ward  and  after  a  longer  or  shorter  journey,  meets  with  a  non- 
porous  layer  of  rock  through  which  it  cannot  pass;  and  which 
effectually  hinders  its  downward  passage.  In  such  regions, 
there  is  an  accumulation  of  water,  and  a  well  dug  there  would 
have  an  abundant  supply  of  water.  The  non-porous  layer  is 
rarely  level,  and  hence  the  water  whose  vertical  path  is  ob- 
structed does  not  "  back  up  "  on  the  soil,  but  flows  down  hill 
parallel  with  the  obstructing  non-porous  layer,  and  in  some 
distant  region  makes  an  outlet  for  itself,  forming  a  spring 
(Fig.  38).  The  streams  originating  in  the  springs  flow  through 
the  land  and  eventually  join  larger  streams  or  rivers ;  from 
the  surface  of  streams  and  rivers  evaporation  occurs,  the 
water  once  more  becomes  vapor  and  passes  into  the  atmos- 
phere, where  it  is  condensed  and  again  falls  to  the  earth. 

Water  which  has  filtered  through  many  feet  of  earth  is  far 
purer  and  safer  than  that  which  fell  directly  into  the  rivers,  or 
which  ran  off  from  the  land  and  joined  the  surface  streams 
without  passing  through  the  soil. 

72.  The  Composition  of  Water.  Water  was  long  thought 
to  be  a  simple  sub- 
stance, but  toward  the 
end  of  the  eighteenth 
century  it  was  found 
to  consist  of  two  quite 
different  substances, 
oxygen  (O)  and  hy- 
drogen (H.) 

If  we  send  an  elec- 
tric current  through 
water  (acidulated  to 
make  it  a  good  con- 
ductor), as  shown  in  Figure  39,  we  see  bubbles  of  gas  rising 
from  the  end  of  the  wire  by  which  the  current  enters  the  water, 


FIG.  39.  —  The  decomposition  of  water. 


80  WATER 

and  other  bubbles  of  gas  rising  from  the  end  of  the  wire  by 
which  the  -current  leaves  the  water.  These  gases  have  evi- 
dently come  from  the  water  and  are  the  substances  of  which 
it  is  composed,  because  the  water  begins  to  disappear  as  the 
gases  are  formed.  If  we  place  over  each  end  of  the  wire  an 
inverted  jar  filled  with  water,  the  gases  are  easily  collected. 
The  first  thing  we  notice  is  that  there  is  always  twice  as  much 
of  one  gas  as  of  the  other ;  that  is,  water  is  composed  of  two 
substances,  one  of  which  is  always  present  in  twice  as  large 
quantities  as  the  other. 

73.  The  Composition  of  Water.     On  testing  the  gases  into 
which  water  is  broken  up  by  an  electric  current,  we  find  them 
to  be  quite  different.     One  proves  to  be  oxygen,  a  substance 
with  which  we  are  already  familiar.     The  other  gas,  hydro- 
gen, is  new  to  us  and  is  interesting  as  being  the  lightest 
known  substance,  being  even  "lighter  than  a  feather." 

An  important  fact  about  hydrogen  is  that  in  burning  it 
gives  as  much  heat  as  five  times  its  weight  of  coal.  Its  flame 
is  blue  and  almost  invisible  by  daylight,  but  intensely  hot. 
If  fine  platinum  wire  is  placed  in  an  ordinary  gas  flame,  it 
does  not  melt,  but  if  placed  in  a  flame  of  burning  hydrogen,  it 
melts  very  quickly. 

74.  How  to  prepare  Hydrogen.     There  are  many  different 
methods  of  preparing  hydrogen,  but  the  easiest  laboratory 
method  is  to  pour  sulphuric  acid,  or  hydrochloric  acid,  on  zinc 
shavings  and  to  collect  in  a  bottle  the  gas  which  is  given  off. 
This  gas  proves  to  be  colorless,  tasteless,  and  odorless.     (See 
Laboratory  Manual.) 


CHAPTER    VII 

AIR 

75.  The  Instability  of  the  Air.     We  are  usually  not  con- 
scious of  the  air  around  us,  but  sometimes  we  realize  that 
the   air  is  heavy,  while  at  other  times  we  feel  the  bracing 
effect  of  the  atmosphere.     We  live  in  an  ocean  of  air  as  truly 
as  fish  inhabit  an  ocean  of  water.     If  you  have  ever  been  at 
the  seashore  you  know  that  the  ocean  is  never  still  for  a  sec- 
ond ;  sometimes  the  waves  surge  back  and   forth   in  angry 
fury,  at  other  times  the  waves  glide  gently  in  to  the  shore 
and  the  surface  is  as  smooth  as  glass  ;  but  we  know  that  there 
is  perpetual  motion  of  the  water  even  when  the  ocean  is  in  its 
gentlest  moods.     Generally  our  atmosphere  is  quiet,  and  we 
are  utterly  unconscious  of  it ;  at  other  times  we  are  painfully 
aware  of  it,  because  of  its  furious  winds.     Then  again  we  are 
oppressed  by  it  because  of  the  vast  quantity  of  vapor  which  it 
holds  in  the  form  of  fog,  or  mist.     The  atmosphere  around 
us  is  as  restless  and  varying  as  is  the  water  of  the  sea.     The 
air  at  the  top  of  a  high  tower  is  very  different  from  the  air  at 
the  base  of  the  tower.     Not  only  does  the  atmosphere  vary 
greatly  at  different  altitudes,  but  it  varies  at  the  same  place 
from  time  to  time,  at  one  period  being  heavy  and  raw,  at 
another  being  fresh  and  invigorating. 

Winds,  temperature,  and  humidity  all  have  a  share  in  deter- 
mining atmospheric  conditions,  and  no  one  of  these  plays  a 
small  part. 

76.  The  Character   of   the   Air.     The   atmosphere   which 
envelops  us  at  all  times  extends  more  than  fifty  miles  above 
us,  its  height  being  far  greater  than  the  greatest  depths  of  the 

CL.    GEN.    SCI.  —  6  8l 


AIR 


sea. 


This  atmosphere  varies  from  place  to  place ;  at  the  sea 
level  it  is  heavy,  on  the  mountain 
top  less  heavy,  and  far  above  the 
earth  it  is  so  light  that  it  does  not 
contain  enough  oxygen  to  permit  man 
to  live.  Figure  40  illustrates  by  a  pile 
of  pillows  how  the  pressure  of  the  air 
varies  from  level  to  level. 

Sea  level  is  a  low  portion  of  the 
earth's  surface,  hence  at  sea  level  there 
is  a  high  column  of  air,  and  a  heavv 

FIG.  40.  —  To  illustrate  the  de-     . 

crease  in  pressure  with  height,  air  pressure.     As  one  passes  from  sea 
level  to  mountain  top   a   gradual  but 

steady  decrease  in  the  height  of  the  air  column  occurs, 
and  hence  a  gradual  but  definite  lessening  of  the  air  pressure. 
77.  Air  Pressure.  If  an  empty  tube  (Fig.  41)  is  placed 
upright  in  water,  the  water  will  not  rise  in  the  tube,  but  if  the 
tube  is  put  in  water  and  the  air  is  then 
drawn  out  of  the  tube  by  the  mouth,  the 
water  will  rise  in  the  tube  (Fig.  42).  This 
is  what  happens  when  we  take  lemonade 
through  a  straw.  When  the  air  is  with- 
drawn from  the  straw  by  the  mouth,  the 
pressure  within  the  straw  is  reduced,  and 
the  liquid  is  forced  up  the  straw  by  the  air 
pressure  on  the  surface  of  the  liquid  in 
the  glass.  Even  the  ancient  Greeks  and 
Romans  knew  that  water  would  rise  in  a  FIG.  41.— The  water  in 
tube  when  the  pressure  within  the  tube  was 
reduced,  and  hence  they  tried  to  obtain 
water  from  wells  in  this  fashion,  but  the  water  could  never 
be  raised  higher  than  32  feet.  Let  us  see  why  water  could 
rise  32  feet  and  no  more.  If  an  empty  pipe  is  placed  in  a 


AIR  PRESSURE 


the  tube  when  the  air 
is  withdrawn. 


cistern  of  water,  the  water  in  the  pipe  does  not  rise  above  the 

level  of  the  water  in  the  cistern.  If,  how- 
ever, the  pressure  in  the  tube  is  removed, 

the  water  in  the  tube  will  rise  to  a  height 

of  32  feet  approximately.     If  now  the  air 

pressure  in  the  tube  is  restored,  the  water 

in  the  tube  sinks  again  to  the  level  of  that 

in  the  cistern.     The  air  pressing  on  the 

liquid  in  the  cistern  tends  to  push  some 

liquid  up  the  tube,  but  the  air  pressing  on 

the  water  in  the  tube  pushes  downwards, 

and  tends  to  keep  the  liquid  from  rising, 

and  these  two  pressures  balance  each  other.   FJG  ^^vvater  rises  in 

When,  however,  the  pressure  within  the 

tube  is  reduced,  the  liquid  rises  because  of 

the  unbalanced  pressure  which  acts  on  the  water  in  the  cistern. 
The  column  of  water  which  can  be  raised  this  way  is  ap- 
proximately 32  feet,  sometimes  a  trifle  more,  sometimes  a 

trifle  less.  If  water  were  twice  as 
heavy,  just  half  as  high  a  column 
could  be  supported  by  the  atmos- 
phere. Mercury  is  about  thirteen 
times  as  heavy  as  water  and,  there- 
fore, the  column  of  mercury  sup- 
ported by  the  atmosphere  is  about 
one  thirteenth  as  high  as  the  column 
of  water  supported  by  the  atmosphere. 
This  can  easily  be  demonstrated. 

FIG.  43.  —  The  air  supports  a   Fill  a  glass  tube  about  a  yard  long 

column  of  mercury  29  inches    ^^  mercuryj  dose  the  open  end  with 

a  finger,  and  quickly  insert  the  end  of 

the  inverted  tube  in  a  dish  of  mercury  (Fig.  43).     When  the  fin- 
ger is  removed,  the  mercury  falls  somewhat,  leaving  an  empty 


84  AIR 

space  in  the  top  of  the  tube.  If  we  measure  the  column  in 
the  tube,  we  find  its  height  is  about  one  thirteenth  of  32  feet 
or  29  inches,  exactly  what  we  should  expect.  Since  there  is 
no  air  pressure  within  the  tube,  the  atmospheric  pressure  on 
the  mercury  in  the  dish  is  balanced  solely  by  the  mercury 
within  the  tube,  that  is,  by  a  column  of  mercury  29  inches  high. 
The  shortness  of  the  mercury  column  as  compared  with  that 
of  water  makes  the  mercury  more  convenient  for  both  experi- 
mental and  practical  purposes.  (See  Laboratory  Manual.) 

78.  The  Barometer.  Since  the  pressure  of  the  air  changes 
from  time  to  time,  the  height  of  the  mercury  will  change  from 
day  to  day,  and  hour  to  hour.  When  the  air  pressure  is  heavy, 
the  mercury  will  tend  to  be  high  ;  when  the  air  pressure  is  low, 
the  mercury  will  show  a  shorter  column ;  and  by  reading  the 
level  of  the  mercury  one  can  learn  the  pressure  of  the  atmos- 
-phere.  If  a  glass  tube  and  dish  of  mercury  are  at- 
tached to  a  board  and  the  dish  of  mercury  is  inclosed 
in  a  case  for  protection  from  moisture  and  dirt,  and 
further  if  a  scale  of  inches  or  centimeters  is  made 
on  the  upper  portion  of  the  board,  we  have  a  mercu- 
rial barometer  (Fig.  44). 

If  the  barometer  is  taken  to  the  mountain  top, 
the  column  of  mercury  falls  gradually  during  the 
ascent,  showing  that  as  one  ascends,  the  pressure 
decreases  in  agreement  with  the  statement  in  Section 
76.  Observations  similar  to  these  were  made  by 
Torricelli  as  early  as  the  sixteenth  century.  Taking 
a  barometric  reading  consists  in  measuring  the 
FIG  4  _A  neight  of  the  mercury  column. 

simple  ba-      79.   A   Portable   Barometer.      The   mercury    ba- 
rometer. .        •       ,   .  .  r 

rometer  is   large  and  inconvenient   to    carry  trom 

place  to  place,  and  a  more  portable  form  has  been  devised, 
known  as  the  aneroid  barometer  (Fig.  45).  This  form  of 


A   PORTABLE  BAROMETER 


FlG.  45.  —  Aneroid  barometer. 


barometer  is  extremely  sensitive ;  indeed,  it  is  so  delicate  that 
it  shows  the  slight  difference  between  the  pressure  at  the  table 
top  and  the  pressure  at  the  floor  level, 
whereas  the  mercury  barometer  would 
indicate  only  a  much  greater  variation 
in  atmospheric  pressure.  The  aneroid 
barometers  are  frequently  made  no 
larger  than  a  watch  and  can  be  carried 
conveniently  in  the  pocket,  but  they 
get  out  of  order  easily  and  must  be  fre- 
quently readjusted.  The  aneroid  ba- 
rometer is  an  air-tight  box  whose  top 
is  made  of  a  thin  metallic  disk  which 
bends  inward  or  outward  according 
to  the  pressure  of  the  atmosphere.  If  the  atmospheric  pressure 
increases,  the  thin  disk  is  pushed  slightly  inward ;  if,  on  the 
other  hand,  the  atmospheric  pressure  decreases,  the  pressure  on 
the  metallic  disk  decreases  and  the  disk  is  not  pressed  so  far 
inward.  The  motion  of  the  disk  is  small,  and  it  would  be  impos- 
sible to  calculate  changes  in  atmospheric 
pressure  from  the  motion  of  the  disk, 
without  some  mechanical  device  to  make 
the  slight  changes  in  motion  perceptible. 

In  order  to  magnify  the  slight  changes 
in  the  position  of  the  disk,  the  thin  face 
is  connected  with  a  system  of  levers,  or 
wheels,  which  multiplies  the  changes  in 
motion  and  communicates  them  to  a 
pointer  which  moves  around  a  graduated 
circular  face.  In  Figure  45  the  real  ba- 
rometer is  scarcely  visible,  being  securely 
inclosed  in  a  metal  case  for  protection  ;  the  principle,  however, 
can  be  understood  by  reference  to  Figure  46. 


FIG.  46.  —  Principle   of 
the  aneroid  barometer. 


86 


AIR 


80.  The  Weight  of  the  Air.     We  have  seen  that  the  pres- 
sure of  the  atmosphere  at  any  point  is  due  to  the  weight  of  the 
air  column  which  stretches  from  that  point  far  up  into  the 
sky  above.     This  weight  varies  slightly  from  time  to  time  and 
from  place  to  place,  but  it  is  equal  to  about  15  pounds  to  the 
square  inch  as  shown  by  actual  measurement.     It  comes  to 
us  as  a  surprise  sometimes  that  air  actually  has  weight ;  for 
example,  a  mass  of   12  cubic  feet  of  air  at  average  pressure 
weighs  i  pound,  and  the  air  in  a  large  assembly  hall  weighs 
more  than  I  ton. 

We  are  practically  never  conscious  of  this  really  enormous 
pressure  of  the  atmosphere,  which  is  exerted  over  every  inch 
of  our  bodies,  because  the  pressure  is  exerted  equally  over 
the  outside  and  the  inside  of  our  bodies  ;  the  cells  and  tissues 
of  our  bodies  containing  gases  under  atmospheric  pressure. 
If,  however,  the  ringer  is  placed  over  the  open  end  of  a  tube 
and  the  air  is  sucked  out  of  the  tube  by  the  mouth,  the  flesh 
of  the  finger  bulges  into  the  tube  because  the  pressure  within 

the  finger  is  no  longer  equalized 
by  the  usual  atmospheric  pressure 
(Fig.  47)- 

Aeronauts  have  never  ascended 
much  higher  than  7  miles  ;  at  that 
height  the  barometer  stands  at  7 
inches  instead  of  at  30  inches,  and 
the  internal  pressure  in  cells  and 
tissues  is  not  balanced  by  an  equal 
external  pressure.  The  unequal- 
ized  internal  pressure  forces  the 
blood  to  the  surface  of  the  body 

and  causes  rupture  of  blood  vessels  and  other  physical  diffi- 
culties. 

81.  Use  of  the  Barometer.     Changes  in  air  pressure  are 


FIG.  47.—  The  flesh  bulges  out. 


USE  OF  THE  BAROMETER  87 

very  closely  connected  with  changes  in  the  weather.  The 
barometer  does  not  directly  foretell  the  weather,  but  a  low  or 
falling  pressure,  accompanied  by  a  simultaneous  fall  of  the 
mercury,  usually  precedes  foul  weather,  while  a  rising  pres- 
sure, accompanied  by  a  simultaneous  rise  in  the  mercury, 
usually  precedes  fair  weather.  The  barometer  is  not  an  in- 


FlG.  48.  —  Barograph. 

fallible  prophet,  but  it  is  of  great  assistance  in  predicting  the 
general  trend  of  the  weather.  There  are  certain  changes  in 
the  barometer  which  follow  no  known  laws,  and  which  allow 
of  no  safe  predictions,  but  on  the  other  hand,  general  future 
conditions  can  be  fairly  accurately  determined.  Figure  48 
shows  a  barograph  or  self-registering  barometer  which  auto- 
matically registers  air  pressure. 

Seaport  towns  in  particular,  but  all  cities,  large  or  small, 
and  villages  too,  are  on  request  notified  by  the  United  States 
Weather  Bureau  ten  hours  or  more  in  advance,  of  probable 
weather  conditions,  and  in  this  way  precautions  are  taken 
which  annually  save  millions  of  dollars  and  hundreds  of  lives. 


88 


AIR 


I  recollect  a  summer  spent  on  a  New  Hampshire  farm;  and 
know  that  an  old  farmer  started  his  farm  hands  haying  by 
moonlight  at  two  o'clock  in  the  morning,  because  the  Special 
Farmer's  Weather  Forecast  of  the  preceding  evening  had 
predicted  rain  for  the  following  day.  His  reliance  on  the 
weather  report  was  not  misplaced,  since  the  storm  came  with 
full  force  at  noon.  Sailing  vessels,  yachts,  and  fishing  dories 
remain  within  reach  of  port  if  the  barometer  foretells  storms. 

82.  Isobaric  and  Isothermal  Lines.  If  a  line  were  drawn 
through  all  points  on  the  surface  of  the  earth  having  an 
equal  barometric  pressure  at  the  same  time,  such  a  line 
would  be  called  an  isobar.  For  example,  if  the  height  of 
barometers  in  different  localities  is  observed  at  exactly  the 
same  time,  and  if  all  the  cities  and  towns  which  have  the 


J80      J60     MO      120      100      80       GO      4O 


0        20        40       60       8< 

—  Isotherms. 


140       160       l&O 


same  pressure  are  connected  by  a  line,  the  curved  lines  will 
be  called  isobars.  By  the  aid  of  these  lines  the  barometric 
conditions  over  a  large  area  can  be  studied.  The  Weather 
Bureau  at  Washington  relies  greatly  on  these  isobars  for 
statements  concerning  local  and  distant  weather  forecasts, 


IV RATHER  MAPS  89 

any  shift  in  isobaric  lines  showing  change  in  atmospheric 
pressure. 

If  a  line  is  drawn  through  all  points  on  the  surface  of  the 
earth  having  the  same  temperature  at  the  same  instant,  such 
a  line  is  called  an  isotherm  (Fig.  49). 

83.  Weather  Maps.  Scattered  over  the  United  States  are 
about  125  Government  Weather  •  Stations,  at  each  of  which 
three  times  a  day,  at  the  same  instant,  accurate  observations 
of  the  weather  are  made.  These  observations,  which  con- 
sist of  the  reading  of  barometer  and  thermometer,  the  deter- 
mination of  the  velocity  and  direction  of  the  wind,  the 
determination  of  the  humidity  and  of  the  amount  of  rain  or 
snow,  are  telegraphed  to  the  chief  weather  official  at  Wash- 
ington. From  the  reports  of  wind  storms,  excessive  rainfall, 
hot  waves,  clearing  weather,  etc.,  and. their  rate  of  travel,  the 
chief  officials  predict  where  the  storms,  etc.,  will  be  at  a  defi- 
nite future  time.  In  the  United  States,  the  general  move- 
ment of  weather  conditions,  as  indicated  by  the  barometer,  is 
from  west  to  east,  and  if  a  certajn  weather  condition  prevails 
in  the  west,  it  is  probable  that  it  will  advance  eastward,  al- 
though with  decided  modifications.  So  many  influences 
modify  atmospheric  conditions  that  unfailing  predictions  are 
impossible,  but  the  Weather  Bureau  predictions  prove  true  in 
about  eight  cases  out  of  ten. 

The  reports  made  out  at  Washington  are  telegraphed  on 
request  to  cities  in  this  country,  and  are  frequently  published 
in  the  daily  papers,  along  with  the  forecast  of  the  local  office. 
A  careful  study  of  these  reports  enables  one  to  forecast  to 
some  extent  the  probable  weather  conditions  of  the  day. 

The  first  impression  of  a  weather  map  (Fig.  50)  with  its 
various  lines  and  signals  is  apt  to  be  one  of  confusion,  and  the 
temptation  comes  to  abandon  the  task  of  finding  an  underly- 
ing plan  of  the  weather.  If  one  will  bear  in  mind  a  few  sim- 


AIR 


BICYCLE  T7RES  gt 

pie  rules,  the  complexity  of  the  weather  map  will  disappear 
and  a  glance  at  the  map  will  give  one  information  concern- 
ing general  weather  conditions  just  as  a  glance  at  the  ther- 
mometer in  the  morning  will  give  some  indication  of  the 
probable  temperature  of  the  day.  (See  Laboratory  Manual.) 

On  the  weather  map  solid  lines  represent  isobars  and 
dotted  lines  represent  isotherms.  The  direction  of  the  wind 
at  any  point  is  indicated  by  an  arrow  which  flies  with  the 
wind ;  and  the  state  of  the  weather  —  clear,  partly  cloudy, 
cloudy,  rain,  snow,  etc.  —  is  indicated  by  symbols. 

84.  Bicycle  Tires.  We  know  very  well  that  we  cannot  put 
more  than  a  certain  amount  of  water  in  a  tube,  but  we  know 
equally  well  that  the  amount  of  air  which  can  be  pumped 
into  a  bicycle  or  automobile  tire  depends  largely  upon  our 
muscular  energy.  A  gallon  of  water  remains  a  gallon  of 
water  and  requires  a  perfectly  definite  amount  of  space,  but 
air  can  be  compressed  and  compressed,  and  made  to  occupy 
less  and  less  space.  While  it  is  true  that  air  is  easily  com- 
pressed, it  is  also  true  that  air  is  elastic  and  capable  of  very 
rapid  and  easy  expansion.  If  a  puncture  occurs  in  a  tire,  the 
compressed  air  escapes  very 
quickly;  that  is,  the  com- 
pressed air  within  the  tube 
has  taken  the  first  opportu- 
nity offered  for  expansion. 

The  fact  that  air  is  elastic 
has  added  materially  to  the 
comfort  of  the  world.  Trans- 
portation by  bicycles  and  au- 
tomobiles has  been  greatly  FIG.  51.  -  By  squeezing  the  bulb,  air  is 

'  forced  out  of  the  nozzle. 

facilitated  by    the  use  of  air 

tires.     In  many  hospitals,  air  mattresses  are  used  in  place  of 

hair,  feather,  or  cotton  mattresses,  and  in  this  way  the  bed  is 


AIR 


kept  fresher  and  cleaner,  and  can  be  moved  with  less  danger  of 
discomfort  to  the  patient.  Every  time  we  squeeze  the  bulb 
of  an  atomizer  we  force  compressed  or  condensed  air  through 
the  atomizer,  and  the  condensed  air  pushes  the  liquid  out  of 
the  nozzle  (Fig.  51).  Thus  we  see  that  in  the  necessities  and 
conveniences  of  life  compressed  air  plays  an  important  part. 

85.  The  Danger  of  Compression.  Air  under  ordinary  atmos- 
pheric conditions  exerts  a  pressure  of  15  pounds  to  the 
square  inch.  If,  now,  large  quantities  of  air  are  compressed 
into  a  small  space,  the  pressure  exerted  becomes  correspond- 
ingly greater.  If  too  much  air  is  blown  into  a  toy  balloon, 
the  balloon  bursts  because  it  cannot  support  the  great  pres- 
sure exerted  by  the  compressed  air  within.  What  is  true  of 
air  is  true  of  all  gases.  Dangerous  boiler  explosions  some- 
times occur  because  the  walls  were  not  strong  enough  to  with- 
stand the  pressure  of  the  steam  (which  is  water  in  the  form 
of  gas).  The  pressure  within  the  boilers 
of  engines  is  frequently  as  much  as  240 
pounds  to  the  square  inch,  and  such  a 
high  pressure  needs  a  boiler  of  strong 
construction. 

86.  How  Pressure  is  Measured  in 
Buildings.  It  was  made  clear  in  the  pre- 
ceding Section  that  undue  pressure  of 
a  gas  may  cause  explosion.  It  is  im- 
portant, therefore,  that  authorities  keep 
strict  watch  on  gases  confined  within 
pipes  and  reservoirs,  never  allowing  the 
pressure  to  exceed  that  which  the  walls 
of  the  reservoir  will  safely  bear. 

Pressure  in  a  gas  pipe  is  measured  in 
the  following  way  by  a  very  simple  instrument  called  the 
pressure  gauge :  The  gauge  consists  of  a  bent  glass  tube 


FIG.    52.  —  A     pressure 
gauge. 


THE  GAS  METER 


93 


containing  mercury,  and  so  made  that  one  end  can  be  fitted 
to  a  gas  jet  (Fig.  52).  When  the  gas  cock  is  closed,  the 
mercury  stands  at  the  same  level  in  both  arms,  but  when  the 
cock  is  opened,  the  gas  whose  pressure  is  being  measured 
forces  the  mercury  up  the  opposite  arm.  If  the  pressure  of 
the  gas  is  small,  the  mercury  changes  its  level  but  very  little. 
It  is  clear  that  the  height  of  a  column  of  mercury  is  a  measure 
of  the  gas  pressure. 

By  actual  measurement  it  has  been  found  that  one  cubic 
inch  of  mercury  weighs  about  half  a  pound.  Hence  a  column 
of  mercury  one  inch  high  indicates  a  pressure  of  about  one 
half  pound  to  the  square  inch ;  a  column  two  inches  high  in- 
dicates a  pressure  of  about  one  pound  to  the  square  inch,  and 
so  on. 

This  is  a  very  convenient  way  to  measure  the  pressure  of 
the  illuminating  gas  in  our  homes  and  offices.  The  gauge  is 
attached  to  the  gas  burner  and  the  pressure  read  by  means  of 
a  scale  attached  to  the  gauge.  (See  Laboratory  Manual.) 

In  order  to  have  satisfactory  illumination,  the  pressure 
must  be  strong  enough  to  give  a  steady,  broad  flame.  If  the 
flame  from  any  gas  jet  is 
flickering  and  weak,  it  is 
usually  an  indication  of 
insufficient  pressure  and 
the  gas  company  should 
investigate  conditions  and 
see  to  it  that  the  con- 
sumer receives  his  proper 
value. 

87.     The      Gas     Meter. 
Most      householders      are 


100  THOUSAND   10  THOUSAND 


1  THOUSAND 


FlG.  53.  —  The  gas  meter  indicates  the  num- 
ber of  cubic  feet  of  gas  consumed. 


deeply  interested  in  the  actual  amount  of  gas  consumed  (gas 
is  charged  for  according  to  the  number  of  cubic  feet  used), 


94 

and  therefore  should  be  able  to  read  the  gas  meter  which  in- 
dicates their  consumption  of  gas.  Such  gas  meters  are  fur- 
nished by  the  companies,  and  can  be  read  with  little  effort. 

The  instrument  itself  is  somewhat  complex.  It  will  suffice 
to  say  that  within  the  meter  box  are  thin  disks  which  are 
moved  by  the  stream  of  gas  that  passes  them.  This  move- 
ment of  the  disks  is  recorded  by  clockwork  devices  on  a  dial 
face.  In  this  way,  the  number  of  cubic  feet  of  gas  which 
pass  through  the  meter  is  automatically  registered.  (See 
Laboratory  Manual.) 

88.  Water  Pressure.  A  pressure  gauge  of  somewhat  dif- 
ferent form  may  also  be  used  to  measure  the  pressure  of 
water.  If  the  gauge  is  attached  to  a  faucet  and  the  faucet  is 
opened,  the  pressure  of  the  water  is  indicated  by  the  rise  of 
the  mercury  as  in  Section  86.  The  city  or  town  is  under 
obligation  to  furnish  the  householder  with  water  under  a  pres- 
sure sufficient  to  give  a  strong,  rapid  flow,  and  if  the  supply 
from  faucets  becomes  feeble  and  slow,  the  pressure  is  below 
its  proper  value. 

In  summer  when  prolonged  drought  occurs,  the  pressure 
falls  considerably,  but  for  this  no  one  can  be  held  directly 
responsible. 


CHAPTER    VIII 


GENERAL   PROPERTIES   OF   GASES 

89.    The  Relation  between  Pressure  and  Volume.     It  was 

long  known  that  as  the  pressure  of  a  gas  increases,  that  is,  as 
it  becomes  compressed,  its  volume  decreases,  but  Robert 
Boyle  was  the  first  to  determine  the  exact  relation  between 
the  volume  and  the  pressure  of  a  gas.  He  did  this  by  means 
of  the  following  simple  experiment  (Fig.  54).  Let  mercury 
be  poured  in  a  U-shaped  tube  until  the 
level  of  the  mercury  in  the  closed  end  of 
the  tube  is  the  same  as  the  level  of  the 
mercury  in  the  open  end.  The  mercury 
just  balances  itself,  and  hence  the  air  col- 
umn in  the  closed  end  must  be  balanced 
by  the  atmospheric  pressure  on  the  open 
end ;  the  amount  of  the  atmospheric  pres- 
sure can  be  easily  determined  by  reading 
the  barometer.  If  the  atmospheric  pres- 
sure as  registered  by  the  barometer  is  30 
inches  or  x  inches,  pour  into  the  long  arm 
enough  mercury  to  make  the  column  D 
equal  to  30  or  x  inches.  The  air  column 
b  is  now  under  a  pressure  equal  to  that  of 
two  atmospheres,  one  atmosphere  of  pres- 
sure being  due  to  mercury,  the  other  atmosphere  of  pressure 
being  due  to  the  weight  of  the  atmosphere  itself.  If  now  the 
air  column  in  the  closed  end  is  measured,  its  volume  will  be 
only  one  half  of  its  former  volume.  By  doubling  the  pres- 

95 


FIG.  54.  —  As  the  pres- 
sure on  the  gas  in- 
creases, its  volume 
decreases. 


96  GENERAL   PROPERTIES   OF  GASES 

sure  we  have  reduced  the  volume  one  half ;  similarly,  if  the 
pressure  is  increased  threefold,  the  volume  will  be  reduced  to 
one  third  of  its  original  volume.  (See  Laboratory  Manual.) 

The  pressure  P  of  a  gas  at  constant  temperature  is  in- 
versely proportional  to  the  volume  V  of  the  gas.  This  can 
be  conveniently  stated  in  another  way  as  follows :  If  P  and 
P'  denote  the  pressures  corresponding  to  the  volumes  V  and 

y,  then 

PV=  P'V 

90.  Heat  due  to  Compression.  We  saw  in  Section  89 
that  whenever  the  pressure  exerted  upon  a  gas  is  increased, 
the  volume  of  the  gas  is  decreased ;  whenever  the  pressure 
upon  a  gas  is  decreased,  the  volume  of  the  gas.is 
increased.  If  the  pressure  is  changed  very  slowly, 
the  change  in  the  temperature  of  the  gas  is  im- 
perceptible ;  if,  however,  the  pressure  is  removed 
suddenly,  the  temperature  falls  rapidly,  or  if  the 
pressure  is  applied  suddenly,  the  temperature  rises 
rapidly.  When  bicycle  tires  are  being  inflated,  the 
pump  b.ecomes  hot  because  of  the  compression  of 
the  air.  The  heat  due  to  compression  can  be 
shown  very  strikingly  by  means  of  the  fire  syringe. 
Dip  a  piece  of  absorbent  cotton  in  carbon  bisul- 
phide, and  shake  the  cotton  in  a  tube  (Fig.  55)  so 
that  a  small  amount  of  the  carbon  bisulphide 

Vfeheatof  gets  mto  tne  tube»  tnen  insert:  the  piston  and 
compression  compress  the  air  suddenly.  So  high  temperature 

is    sufficient     .       ,         ,  111  ,  ,  .  r      *          • 

to  ignite  the  1S  developed  by  the  sudden  compression  of  the  air 
carbon  bi-  that  a  flash  f  rom  the  carbon  bisulphide  is  produced. 

sulphide. 

The  amount  of  heat  resulting  from  compres- 
sion is  surprisingly  large ;  for  example,  if  a  mass  of  gas  at 
o°  C.  is  suddenly  compressed  to  one  half  its  original  volume, 
its  temperature  rises  87°  C. 


UNEXPECTED    TRANSFORMATIONS  97 

91.  Cooling  by  Expansion.     If  a  gas  expands  suddenly,  its 
temperature  falls ;  for  example,  if  a  mass  of  gas  at  87°  C.  is 
allowed  to   expand  rapidly  to  twice  its   original   volume,  its 
temperature  falls  to  o°  C.     If  the  compressed  air  of  a  bicycle 
tire  is  allowed  to  expand  and  a  sensitive  thermometer  is  held 
in  the  path  of  the  escaping  air,  the  thermometer  will  show  a 
decided  drop  in  temperature. 

The  low  temperature  obtained  by  the  expansion  of  air  or 
other  gases  is  utilized  commercially  on  a  large  scale.  By 
means  of  powerful  pistons  air  is  compressed  to  one  third  or 
one  fourth  its  original  volume,  passed  through  a  coil  of 
pipe  surrounded  with  cold  water,  and  then  allowed  to 
escape  into  large  refrigerating  vaults,  which  thereby  have 
their  temperatures  noticeably  lowered,  and  can  be  used  for 
the  permanent  storage  of  meats,  fruits,  and  other  perishable 
material.  In  summer,  when  the  atmospheric  temperature  is 
high,  the  storage  and  preservation  of  foods  is  of  vital  impor- 
tance to  factories  and  cold  storage  houses,  and  but  for  the  low 
temperature  obtainable  by  the  expansion  of  compressed  gases, 
much  of  our  food  supply  would  be  lost  to -use. 

92.  Unexpected  Transformations.     If  the  pressure  on  a  gas 
is    greatly    increased,    a    sudden    transformation    sometimes 
occurs  and  the  gas  becomes  a  liquid.     Then,  if  the  pressure 
is  reduced,  a  second  transformation  occurs,  and  the  liquid 
evaporates  or  returns  to  its  original  form  as  a  gas. 

In  Section  23  we  saw  that  a  fall  of  temperature  caused 
water  vapor  to  condense  or  liquefy.  If  temperature  alone  were 
considered,  most  gases  could  not  be  liquefied,  because  the  tem- 
perature at  which  the  average  gas  liquefies  is  so  low  as  to  be 
out  of  the  range  of  possibility;  it  has  been  calculated,  for 
example,  that  a  temperature  of  252°  C.  below  zero  would  have 
to  be  obtained  in  order  to  liquefy  hydrogen. 

Some  gases  can  be  easily  transformed  into  liquids  by  pres- 
CL.  GEN.  sci.  —  7 


98 


GENERAL   PROPERTIES   OF  GASES 


sure  alone,  some  gases  can  be  easily  transformed  into  liquids 
by  temperature  alone ;  on  the  other  hand,  many  gases  are  so 
difficult  to  liquefy  that  both  pressure  and  low  temperature  are 
needed  to  produce  the  desired  result.  If  a  gas  is  cooled  and 
compressed  at  the  same  time,  liquefaction  occurs  much  more 
surely  and  easily  than  though  either  factor  alone  were  de- 
pended upon.  The  air  which  surrounds  us,  and  of  whose  ex- 
istence we  are  scarcely  aware,  can  be  reduced  to  the  form  of 
a  liquid,  but  the  pressure  exerted  upon  the  portion  to  be  lique- 
fied must  be  thirty-nine  times  as  great  as  the  atmospheric 
pressure  and  the  temperature  must  have  been  reduced  to  a 
very  low  point. 

93.  Artificial  Ice.  Ammonia  gas  is  liquefied  by  strong  pres- 
sure and  low  temperature  and  is  then  allowed  to  flow  into  pipes 
which  run  through  tanks  containing  salt  water.  The  reduction 
of  pressure  causes  the  liquid  to  evaporate  or  turn  to  a  gas, 
and  the  fall  of  temperature  which  always  accompanies  evapo- 
ration means  a  lowering  of  the  temperature  of  the  salt  water 
to  1 6  or  1 8°  below  zero.  The  brine  does  not  freeze,  however, 
because  it  is  kept  in  constant  motion  and  has  a  low  freezing 
point.  But  immersed  in  the  salt  water  are  molds  containing 
pure  water,  and  since  the  freezing  point  of  water  is  o°  C.,  the 


FIG.  56.  —  Apparatus  for  making  artificial  ice. 

water  in  the  molds  freezes  and  can  be  drawn  from  the  mold  as 
solid  cakes  of  ice. 


ARTIFICIAL   ICE  99 

Ammonia  gas  is  driven  by  the  pump  C  into  the  coil  D 
(Fig.  56)  under  a  pressure  strong  enough  to  liquefy  it,  the 
heat  generated  by  this  compression  being  carried  off  by  cold 
water  which  constantly  circulates  through  B.  The  liquid 
ammonia  flows  through  the  regulating  valve  Finto  the  coil  E, 
in  which  the  pressure  is  kept  low  by  the  pump  C.  The  ac- 
companying expansion  reduces  the  temperature  to  a  very  low 
degree,  and  the  brine  which  circulates  around  the  coil  E  ac- 
quires a  temperature  below  the  freezing  point  of  pure  water. 
The  cold  brine  passes  from  A  to  a  tank  in  which  are  im- 
mersed cans  filled  with  water,  and  within  a  short  time  the 
water  in  the  cans  is  frozen  into  solid  cakes  of  ice. 


CHAPTER    IX 

INVISIBLE  OBJECTS 

94.  Very  Small  Objects.  We  saw  in  Section  84  that  gases 
have  a  tendency  to  expand,  but  that  they  can  be  compressed 
by  the  application  of  force.  This  observation  has  led  scien- 
tists to  suppose  that  substances  are  composed  of  very  minute 
particles  called  molecules,  separated  by  small  spaces  called 
pores ;  and  that  when  a  gas  is  condensed,  the  pores  become 
smaller,  and  that  when  a  gas  expands,  the  pores  become  larger. 

The  fact  that  certain  substances  are  soluble,  like  sugar  in 
water,  shows  that  the  molecules  of  sugar  find  a  lodging  place 
in  the  spaces  or  pores  between  the  molecules  of  water,  in 
much  the  same  way  that  pebbles  find  lodgment  in  the  chinks 
of  the  coal  in  a  coal  scuttle.  An  indefinite  quantity  of  sugar 
cannot  be  dissolved  in  a  given  quantity  of  liquid,  because 
after  a  certain  amount  of  sugar  has  been  dissolved  all  the 
pores  become  filled,  and  there  is  no  available  molecular  space. 
The  remainder  of  the  sugar  settles  at  the  bottom  of  the  ves- 
sel, and  cannot  be  dissolved  by  any  amount  of  stirring. 

If  a  piece  of  potassium  permanganate  about  the  size  of  a 
grain  of  sand  is  put  into  a  quart  of  water,  the  solid  disappears 
and  the  water  becomes  a  deep  rich  red.  The  solid  evidently 
has  dissolved  and  has  broken  up  into  minute  particles  which 
are  too  small  to  be  seen,  but  which  have  scattered  themselves 
and  lodged  in  the  pores  of  the  water,  thus  giving  the  water 
its  rich  color. 

100 


JOURNEYS  MADE  BY  MOLECULES  lOI 

There  is  no  visible  proof  of  the  existence  of  molecules 
and  molecular  spaces,  because  not  only  are  our  eyes  unable 
to  see  them  directly,  but  even  the  most  powerful  microscope 
cannot  make  them  visible  to  us.  They  are  so  small  that  if 
one  thousand  of  them  were  laid  side  by  side,  they  would 
make  a  speck  too  small  to  be  seen  by  the  eye  and  but  barely 
visible  under  a  powerful  microscope. 

We  cannot  see  molecules  or  molecular  pores,  but  the  phe- 
nomena of  compression  and  expansion,  solubility  and  other 
equally  convincing  facts,  have  led  us  to  conclude  that  all 
substances  are  composed  of  very  minute  particles  or  mole- 
cules separated  by  spaces  called  pores. 

95.  Journeys  Made  by  Molecules.  If  a  gas  jet  is  turned 
on  and  not  lighted,  an  odor  of  gas  soon  becomes  perceptible, 
not  only  throughout  the  room,  but  in  adjacent  halls  and  even 
in  distant  rooms.  An  uncorked  bottle  of  cologne  scents  an 
entire  room,  the  odor  of  a  rose  or  violet  permeates  the 
atmosphere  near  and  far.  These  simple  everyday  occur- 
rences seem  to  show  that  the  molecules  of  a  gas  must  be  in  a 
state  of  continual  and  rapid  motion.  In  the  case  of  the 
cologne,  some  molecules  must  have  escaped  from  the  liquid 
by  the  process  of  evaporation  and  traveled  through  the  air  to 
the  nose.  We  know  that  the  molecules  of  a  liquid  are  in 
motion  and  are  continually  passing  into  the  air  because  in 
time  the  vessel  becomes  empty.  The  only  way  in  which  this 
could  happen  would  be  for  the  molecules  of  the  liquid  to  pass 
from  the  liquid  into  the  surrounding  medium;  but  this  is 
really  saying  that  the  molecules  are  in  motion. 

From  these  phenomena  and  others  it  is  reasonably  clear 
that  substances  are  composed  of  molecules,  and  that  mole- 
cules are  not  inert,  quiet  particles,  but  that  they  are  in  in- 
cessant motion,  moving  rapidly  hither  and  thither,  sometimes 
traveling  far,  sometimes  near.  Even  the  log  of  wood  which 


102  INVISIBLE  OBJECTS 

lies  heavy  and  motionless  on  our  woodpile  is  made  up  of 
countless  billions  of  molecules  each  in  rapid  incessant  motion. 
The  molecules  of  solid  bodies  cannot  escape  so  readily  as 
those  of  liquids  and  gases,  and  do  not  travel  far.  The  log 
lies  year  after  year  in  an  apparently  motionless  condition, 
but  if  one's  eyes  were  keen  enough,  the  molecules  would  be 
seen  moving  among  themselves,  even  though  they  cannot 
escape  into  the  surrounding  medium  and  make  long  journeys 
as  do  the  molecules  of  liquids  and  gases. 

96.  The  Companions  of  Molecules.     Common  sense  tells  us 
that  a  molecule  of  water  is  not  the  same  as  a  molecule  of 
vinegar;  the  molecules  of  each  are  infinitely  small  and  in 
rapid  motion,  but  they  differ  essentially,  otherwise  one  sub- 
stance would  be  like  every  other  substance.     What  is  it  that 
makes  a  molecule  of  water  differ  from  a  molecule  of  vinegar, 
and  each  differ  from  all  other  molecules  ?     Strange  to  say,  a 
molecule  is  not  a  simple  object,  but  is  quite  complex,  being 
composed  of  one  or  more  smaller  particles,  called  atoms,  and 
the  number  and  kind  of  atoms  in  a  molecule  determine  the 
type  of  the  molecule,  and  the  type  of   the  molecule  deter- 
mines the  substance.     For  example,  a  glass  of  water  is  com- 
posed of  untold  millions  of  molecules,  and  each  molecule  is 
a  company  of  three  still  smaller  particles,  one  of  which  is 
called  the  oxygen  atom  and  two  of  which  are  alike  in  every 
particular  and  are  called  hydrogen  atoms. 

97.  Simple  Molecules.     Generally  molecules  are  composed 
of   atoms  which  are  different  in  kind.      For  example,  the 
molecule  of  water  has  two  different  atoms,  the  oxygen  atom 
and  the  hydrogen  atoms;  alcohol  has  three  different  kinds 
of  atoms,  oxygen,  hydrogen,  and  carbon.     Sometimes,  how- 
ever, molecules   are   composed  of  a  group  of  atoms  all   of 
which  are  alike.     Now  there  are  but  seventy  or  eighty  differ- 
ent kinds  of  atoms,  and  hence  there  can  be  but  seventy  or 


SIM  PIE  MOLECULES  103 

eighty  different  substances  whose  molecules  are  composed  of 
atoms  which  are  alike.  When  the  atoms  comprising  a  mole- 
cule are  all  alike,  the  substance  is  called  an  element,  and  is 
said  to  be  a  simple  substance.  Throughout  the  length  and 
breadth  of  this  vast  world  of  ours  there  are  only  about  eighty 
known  elements.  An  element  is  the  simplest  substance  con- 
ceivable, because  it  has  not  been  separated  into  anything 
simpler.  Water  is  a  compound  substance.  'It  can  be  sepa- 
rated into  oxygen  and  hydrogen. 

Gold,  silver,  and  lead  are  examples  of  elements,  and  water, 
alcohol,  cider,  sand,  and  marble  are  complex  substances,  or 
compounds,  as  we  are  apt  to  call  them.  Everything,  no 
matter  what  its  size  or  shape  or  character,  is  formed  from  the 
various  combinations  into  molecules  of  a  few  simple  atoms,  of 
which  there  exist  about  eighty  known  different  kinds.  Per- 
haps this  can  be  made  clearer  by  comparison  with  the  con- 
struction of  our  own  language.  Every  word,  no  matter  what 
its  size  or  meaning,  is  formed  from  varying  combinations  of 
the  twenty-six  letters  of  the  alphabet.  "  Too "  might  be 
thought  of  as  representing  water,  which  is  composed  of  two 
distinct  kinds  of  atoms,  oxygen  and  hydrogen. 


CHAPTER    X 

LIGHT 

98.  What  Light  Does  for  Us.     Heat  keeps  us  warm,  cooks 
our  food,  drives  our  engines,  and  in  a  thousand  ways  makes 
life  comfortable  and  pleasant,  but  what  should  we  do  without 
light  ?     How  many  of  us  could  be  happy  even  though  warm 
and  well  fed  if  we  were  forced  to  live  in  the  dark  where  the 
sunbeams  never  flickered,  where  the  shadows  never  stole  across 
the  floor,  and  where  the  soft  twilight  could  not  tell  us  that  the 
day  was  done  ?     Heat  and  light  are  the  two  most  important 
physical  factors  in  life ;  we  cannot  say  which  is  the  more  nec- 
essary, because  in  the  extreme  cold  or  arctic  regions  man 
cannot  live,  and  in  the  dark  places  where  the  light  never  pene- 
trates man  sickens  and  dies.     Both  heat  and  light  are  essen- 
tial to  life,  and  each  has  its  own  part  to  play  in  the  varied 
existence  of  man  and  plant  and  animal. 

Light  enables  us  to  see  the  world  around  us,  makes  the 
beautiful  colors  of  the  trees  and  flowers,  enables  us  to  read, 
is  essential  to  the  taking  of  photographs,  gives  us  our  moving 
pictures  and  our  magic  lanterns,  produces  the  exquisite  tints 
of  stained-glass  windows,  and  brings  us  the  joy  of  the  rain- 
bow. We  do  not  always  realize  that  light  is  beneficial,  because 
sometimes  it  fades  our  clothing  and  our  carpets,  arid  burns 
our  skin  and  makes  it  sore.  But  we  shall  see  that  even  these 
apparently  harmful  effects  of  light  are  in  reality  of  great 
value  in  man's  constant  battle  against  disease. 

99.  The  Candle.     Natural  heat  and  light  are  furnished  by 
the  sun,   but   the   absence  of   the   sun  during  the  evening 
makes  artificial  light  necessary,   and  even  during  the  day 

104 


FADING  ILLUMINATION 


105 


artificial  light  is  needed  in  buildings  whose  structure  excludes 
the  natural  light  of  the  sun.  Artificial  light  is  furnished  by 
electricity,  by  gas,  by  oil  in  lamps,  and  in  numerous  other 
ways.  Until  modern  times  candles  were  the  main  source  of 
light,  and  indeed  to-day  the  intensity,  or  power,  of  any  light 
is  measured  in  candle  power  units,  just  as  length  is  measured 
in  yards ;  for  example,  an  average  gas  jet  gives  a  10  candle 
power  light,  or  is  ten  times  as  bright  as  a  candle  ;  an  ordinary 
incandescent  electric  light  gives  a  16  candle  power  light,  or 
furnishes  sixteen  times  as  much  light  as  a  candle.  Very  strong 
large  oil  lamps  can  at  times  yield  a  light  of  60  candle  power, 
while  the  large  arc  lamps  which  flash  out  on  the  street 
corners  are  said  to  furnish  1200  times  as  much  light  as  a 
single  candle.  Naturally  all  candles  do  not  give  the  same 
amount  of  light,  nor  are  all  candles  alike  in  size.  The 
candles  which  decorate  our  tea  tables  are  of  wax,  while 
those  which  serve  for  general  use  are  of  paraffin  and  tallow. 
The  standard  candle  with  which  all  lights  are  compared  is 
made  of  spermaceti  and  has 
a  weight  of  \  of  a  pound, 
and  a  diameter  of  J  of  an 
inch. 

100.  Fading  Illumina- 
tion. The  farther  we  move 
from  a  light,  the  less 
strong,  or  intense,  is  the 
illumination  which  reaches 
us ;  the  light  of  the  street 

lamp  on  the  corner  fades  and  becomes  dim  before  the  middle 
of  the  block  is  reached,  so  that  we  look  eagerly  for  the  next 
lamp.  The  light  diminishes  in  brightness  much  more  rapidly 
than  we  realize,  as  the  following  simple  experiment  will  show. 
Let  a  single  candle  (Fig.  57)  serve  as  our  light,  and  at  a 


FIG.  57.  —  A  photograph   at  a   receives  four 
times  as  much  light  as  when  held  at  b. 


106  LIGHT 

distance  of  one  foot  from  the  candle  place  a  photograph. 
In  this  position  the  photograph  receives  a  definite  amount  of 
light  from  the  candle  and  has  a  certain  brightness. 

If  now  we  place  a  similar  photograph  directly  behind  the 
first  photograph  and  at  a  distance  of  two  feet  from  the  candle, 
the  second  photograph  receives  no  light  because  the  first  one 
cuts  off  all  the  light.  If,  however,  the  first  photograph  is  re- 
moved, the  light  which  fell  on  it  passes  outward  and  spreads 
itself  over  a  larger  area,  until  at  the  distance  of  the  second 
photograph  the  light  spreads  itself  over  four  times  as  large 
an  area  as  formerly.  At  this  distance,  then,  the  illumination 
on  the  second  photograph  is  only  one  fourth  as  strong  as  it 
was  on  a  similar  photograph  held  at  a  distance  of  one  foot 
from  the  candle. 

The  photograph  or  object  placed  at  a  distance  of  one  foot 
from  a  light  is  well  illuminated;  if  it  is  placed  at  a  distance 
of  two  feet,  the  illumination  is  only  one  fourth  as  strong,  and  if 
the  object  is  placed  three  feet  away,  the  illumination  is  only  one 
ninth  as  strong.  This  fact  should  make  us  have  thought  and 
care  in  the  use  of  our  eyes.  We  think  we  are  sixteen  times  as 
well  off  with  our  incandescent  lights  as  our  ancestors  were  with 
simple  candles,  but  we  must  reflect  that  our  ancestors  kept  the 
candle  near  them,  "  at  their  elbow,"  so  to  speak,  while  we  sit  at 
some  distance  from  the  light  and  unconcernedly  read  and  sew. 

As  an  object  recedes  from  a  light  the  illumination  which  it 
receives  diminishes  rapidly,  for  the  strength  of  the  illumina- 
tion is  inversely  proportional  to  the  square  of  distance  of  the 
object  from  the  light.  Our  ancestors  with  a  candle  at  a  dis- 
tance of  one  foot  from  a  book  were  as  well  off  as  we  are  with 
an  incandescent  light  four  feet  away. 

101.  Money  Value  of  Light.  Light  is  bought  and  sold  al- 
most as  readily  as  are  the  products  of  farm  and  dairy ;  many 
factories,  churches,  and  apartments  pay  a  definite  sum  for 


MONEY   VALUE  OF  LIGHT 


107 


electric  light  of  a  standard  strength,  and  naturally  full  value 
is  desired.  An  instrument  for  measuring  the  strength  of 
a  light  is  called  a  photometer,  and  there  are  many  different 
varieties,  just  as  there  are  varieties  of  scales  which  measure 
household  articles.  One  light-measuring  scale  depends  upon 
the  law  that  the  intensity  of  illumination  decreases  with  the 
square  of  the  distance  of  the  object  from  the  light.  Sup- 
pose we  wish  to  measure  the  strength  of  the  electric  light 
bulbs  in  our  homes,  in  order  to  see  whether  we  are  getting 
the  specified  illumination.  In  front  of  a  screen  place  a  black 
rod  (Fig.  58)  which  is  illuminated  by  two  different  lights; 
namely,  a  standard 
candle  and  an  incan- 
descent bulb  whose 
strength  is  to  be  meas- 
ured. Two  shadows  of 
the  rod  will  fall  on  the 
screen,  one  caused  by 
the  candle  and  the  other 
caused  by  the  incandescent  light.  The  shadow  due  to  the  latter 
source  is  not  so  dark  as  that  due  to  the  candle.  .Now  let  the 
incandescent  light  be  moved  away  from  the  screen  until  the 
two  shadows  are  of  equal  brightness  or  darkness.  If  the  in- 
candescent light  is  four  times  as  far  away  from  the  screen  as 
the  candle,  and  the  shadows  are  equal,  we  know,  by  Section 
100,  that  its  strength  is  sixteen  candle  power.  If  the  incan- 
descent light  is  four  times  as  far  away  from  the  screen  as  the 
candle  is,  its  power  must  be  sixteen  times  as  great,  and  we 
know  the  company  is  furnishing  the  standard  amount  of  light 
for  a  sixteen  candle  power  electric  bulb.  If,  however,  the 
bulb  must  be  moved  nearer  in  order  that  the  two  shadows 
may  be  of  equal  brightness,  the  power  of  illumination  is  less 
than  the  standard  contract. 


FIG.  58.  —  The  two  shadows  are  equally  dark. 


108  LIGHT 

102.  How  Light  Travels.     We  never  expect  to  see  around 
a  corner,  and  if  we  wish  to  see  through   pinholes  in  three 
separate  pieces  of  cardboard,  we  place  the  cardboards  so  that 
the  three  holes  are  in  a  straight  line.     When  sunlight  enters 

a  dark  room  through  a 
small  opening,  the  dust 
particles  dancing  in 
the  sun  show  a  straight 

FIG.  59.  —  The  candle  cannot  be  seen  unless  the  .  , 

three  pinholes  are  in  a  straight  line.  ray.       If  a  hole  IS  made 

in  a  card,  and  the  card 

is  held  in  front  of  a  light,  the  card  casts  a  shadow,  in  the  cen- 
ter of  which  is  a  bright  spot.  The  light,  the  hole,  and  the 
bright  spot  are  all  in  the  same  straight  line.  These  simple 
observations  lead  us  to  think  that  light  travels  in  a  straight 
line. 

We  can  always  tell  the  direction  from  which  light  comes, 
either  by  the  shadow  cast  or  by  the  bright  spot  formed  when 
an  opening  occurs  in  the  opaque  object  casting  the  shadow. 
If  the  shadow  of  a  tree  falls  towards  the  west,  we  know  the 
sun  must  be  in  the  east;  if  a  bright  spot  is  on  the  floor,  we 
can  easily  locate  the  light  whose  rays  stream  through  an 
opening  and  form  the  bright  spot.  We  know  that  light 
travels  in  a  straight  line,  and  following  the  path  of  the  beam 
which  comes  to  our  eyes,  we  are  sure  to  locate  the  light. 

103.  Good  and  Bad  Mirrors.     As  we  walk  along  the  street, 
we  frequently  see  ourselves  reflected  in  the  shop  windows,  in 
polished  metal  signboards,  in  the  metal  trimmings  of  wagons 
and  automobiles ;  but  in  mirrors  we  get  the  best  image  of 
ourselves.     We  resent  the  image  given  by  a  piece  of  tin,  be- 
cause the  reflection  is  distorted  and  does  not  picture  us  as  we 
really  are ;  a  rough  surface  does  not  give  a  fair  representa- 
tion ;  if  we  want  a  true  image  of  ourselves,  we  must  use  a 
smooth  surface  like  a  mirror  as  a  reflector.     If  the  water  in  a 


THE  PATH   OF  LIGHT  109 

pond  is  absolutely  still,  we  get  a  clear,  true  image  of  the  trees, 
but  if  there  are  ripples  on  the  surface,  the  reflection  is  blurred 
and  distorted.  A  metal  roof  reflects  so  much  light  that  the 
eyes  are  dazzled  by  it,  and  a  whitewashed  fence  injures  the 
eyes  because  of  the  glare  which  comes  from  the  reflected 
light.  Neither  of  these  could  be  called  mirrors,  however,  be- 
cause although  they  reflect  light,  they  reflect  it  so  irregularly 
that  not  even  a  suggestion  of  an  image  can  be  obtained. 

Most  of  us  are  sufficiently  familiar  with  mirrors  to  know 
that  the  image  is  a  duplicate  of  ourselves  with  regard  to  size, 
shape,  color,  and  expression,  but  that  it  appears  to  be  back  of 
the  mirror,  while  we  are  actually  in  front  of  the  mirror.  The 
image  appears  not  only  behind  the  mirror,  but  it  is  also  ex- 
actly as  far  back  of  the  mirror  as  we  are  in  front  of  it ;  if  we 
approach  the  mirror,  the  image  also  draws  nearer ;  if  we  with- 
draw, it  likewise  recedes. 

104.  The  Path  of  Light.  If  a  mirror  or  any  other  polished 
surface  is  held  in  the  path  of  a  sunbeam,  some  of  the  light  is 
reflected,  and  by  rotating  the  mirror  the  reflected  sunbeam 
may  be  made  to  take  any  path.  School  children  amuse 
themselves  by  reflecting  sunbeams  from  a  mirror  into  their 
companions'  faces.  If  the  companion  moves  his  head  in  order 
to  avoid  the  reflected  beam,  his  tormentor  moves  or  inclines 
the  mirror  and  flashes  the  beam  back  to  his  victim's  face. 

If  a  mirror  is  held  so  that  a  ray  of  light  strikes  it  in  a 
perpendicular  direction,  the  light  is  reflected  backward  along 
the  path  by  which  it  came.  If,  however,  the  light  makes  an 
angle  with  the  mirror,  its  direction  is  changed,  and  it  leaves 
the  mirror  along  a  new  path.  By  observation  we  learn  that 
when  a  beam  strikes  the  mirror  and  makes  an  angle  of  30° 
with  the  perpendicular,  the  beam  is  reflected  in  such  a  way 
that  its  new  path  also  makes  an  angle  of  30°  with  the  per- 
pendicular. If  the  sunbeam  strikes  the  mirror  at  an  angle  of 


no 


LIGHT 


FIG.  60.  —  The  ray  AC  is  reflected  as  CD. 


32°  with  the  'perpendicular,  the  path  of  the  reflected  ray  also 

makes  an  angle  of  32°  with 
the  perpendicular.  The 
ray  (AC,  Fig.  60)  which 
falls  upon  the  mirror  is 
called  the  incident  ray,  and 
the  angle  which  the  inci- 
dent ray  (A  C)  makes  with 
the  perpendicular  (BC}  to 
the  mirror,  at  the  point 
where  the  ray  'strikes  the 
mirror,  is  called  the  angle 
of  incidence.  The  angle 
formed  by  the  reflected  ray 
(CD}  and  this  same  per- 
pendicular is  called  the  angle  of  reflection.  Observation  and 
experiment  have  taught  us  that  light  is  always  reflected  in 
such  a  way  that  the  angle 
of  reflection  equals  the 
angle  of  incidence.  Light 
is  not  the  only  illustration 
we  have  of  the  law  of  re- 
flection. Every  child  who 
bounces  a  ball  makes  use 
of  this  law,  but  he  uses 
it  unconsciously.  If  an 
elastic  ball  is  thrown  per- 
pendicularly against  the 
floor,  it  returns  to  the 
sender;  if  it  is  thrown 

against  the  floor  at  an  angle  (Fig.  61),  it  rebounds  in  the 
opposite  direction,  but  always  in  such  a  way  that  the  angle  of 
reflection  equals  the  angle  of  incidence. 


\ 


FIG.  61.  —  A  bouncing  ball  illustrates  the  law  of 
reflection. 


WHY  OBJECTS  ARE   VISIBLE 


III 


FlG.  62.  —  The  image  is  a  duplicate  of  the 
object,  but  appears  to  be  behind  the 
mirror. 


105.  Why  the  Image  seems  to  be  behind  the  Mirror.     If  a 
candle  is  placed  in  front  of  a  mirror,  as  in  Figure  62,  one  of 
the    rays    of    light    which 

leaves  the  candle  will  fall 

upon  the  mirror  asA£  and 

will  be  reflected  as  BC  (in 

such  a  way  that  the  angle 

of    reflection     equals     the 

angle  of  incidence).     If  an 

observer   stands  at    C,  he 

will  think  that  the  point  A 

of  the  candle  is  somewhere 

along  the  line  CB  extended. 

Such  a  supposition  would  be  justified  from  Section  102.     But 

the  candle  sends  out  light  in  all  directions  ;  one  ray  therefore 

will  strike  the  mirror  as  AD  and  will  be  reflected  as  DE,  and  an 

observer  at  E  will  think  that  the  point  A  of  the  candle  is 

somewhere  along  the  line  ED.     In  order  that  both  observers 

may  be  correct,  that  is,  in  order  that  the  light  may  seem  to  be 

in  both  these  directions,  the  image  of  the  point  A  must  seem 

to  be  at  the  intersection  of  the- two  lines.     In  a  similar  manner 

it  can  be  shown  that  every  point  of  the  image  of  the  candle 

seems  to  be  behind  the  candle. 

It  can  be  shown  by  experiment  that  the  distance  of  the 
image  behind  the  mirror  is  equal  to  the  distance  of  the  object 
in  front  of  the  mirror. 

106.  Why  Objects  are  Visible.     If  the  beam  of  light  falls 
upon  a  sheet  of  paper  instead  of  upon  a  smooth  polished 
surface,  no  definite  reflected  ray  will  be  seen,  but  a  glare  will 
be  produced  by  the  scattering  of  the  beam  of  light.     The 
surface  of  the  paper  is  rough,  and  we  can  think  of  it  as  com- 
posed of  many  tiny  mirrors  inclined  in  every  possible  direc- 
tion.    Each  hump  and  hollow  reflects  its  minute  ray  in  its 


112  LIGHT 

own  way,  and  as  a  result  the  beam  is  scattered  in  every 
direction.  The  innumerable  reflecting  surfaces  of  which  the 
paper  is  composed  (Fig.  63)  cause  irregular  diffuse  reflections 

of  the  beams,  and  hence  the  re- 
flected rays  leave  the  paper  in 
every  conceivable  direction.  It 
is  hard  for  us  to  realize  that  a 
smooth  sheet  of  paper  is  by  no 
means  so  smooth  as  it  looks.  It 
is  rough  compared  with  a  pol- 
ished mirror.  The  law  of  reflec- 


FIG.  63.  -The  surface  of  the  paper,  al-    tion  alwaYS    h°lds>     however,   no 

though  smooth  in  appearance,  is  in  matter  what  the  reflecting   sur- 

reality  rough,  and  scatters  the  light  in    f  ,        r        n 

every  direction.  face  is,  —  the  angle  of  reflection 

always  equals  the  angle  of  inci- 
dence. In  a  smooth  body  the  reflecting  surfaces  are  all  par- 
allel and  reflect  in  the  same  direction ;  in  the  case  of  a  rough 
body,  the  tiny  reflecting  surfaces  are  inclined  to  each  other  in 
all  sorts  of  ways,  and  no  two  beams  are  reflected  in  exactly 
the  same  way. 

Hot  coals,  red-hot  stoves,  gas  flames,  and  candles  shine  by 
their  own  light,  and  are  self-luminous,  while  other  bodies,  like 
chairs,  tables,  carpets,  have  no  light  within  themselves  and  are 
visible  only  when  they  receive  light  from  a  luminous  source 
and  reflect  that  light.  These  objects  are  not  self-luminous 
because  at  night  they  are  visible  only  when  a  lamp  or  gas  is 
burning.  When  light  from  any  luminous  object  falls  upon 
books,  desks,  or  dishes,  it  meets  rough  surfaces,  and  hence 
undergoes  diffuse  reflection,  and  is  scattered  irregularly  in  all 
directions.  No  matter  where  the  eye  is,  some  reflected  rays 
enter  it,  and  the  various  objects  are  clearly  seen. 


CHAPTER    XI 


REFRACTION 

107.    Bent  Rays  of  Light.     A  straw  in  a  glass  of  lemonade 
seems  to  be  broken  at  the  surface  of  the  liquid,  the  handle  of 
a    teaspoon    in    a    cup    of 
water  appears  broken,  and 
objects     seen     through    a 
glass  of  water  may  seem 
distorted   and   changed    in 
size.     When    light   passes 
from  air  into  water,  or  from 
any  transparent  substance 
into   another   of    different 
density,    its     direction     is 
changed,   and   it    emerges 
along  an  entirely  new  path 
(Fig.    64).     We    know  that   light   rays  pass  through  glass, 
because  we  can  see  through  the  window  panes  and  through 
our   spectacles ;    we   know    that    light    rays    pass   through 
water,  because  we  can  see  through  a  glass  of  clear  water; 
on  the  other  hand,  light   rays    cannot    pass  through    wood, 
leather,  metal,  etc. 

Whenever  light  meets  a  transparent  substance  obliquely, 
some  of  it  is  reflected,  undergoing  a  change  in  its  direction ; 
and  some  of  it  passes  onward  through  the  medium,  but  the 
latter  portion  passes  onward  along  a  new  path.  The  ray  RO 
(Fig.  65)  passes  obliquely  through  the  air  to  the  surface  of 

CL.    GEN.    SCI.— 8  IM 


FIG.  64.  —  A  straw  or  stick   in  water  seems 
broken. 


114 


REFRACTION 


S      P 
FIG.  65.—  When  the  ray  RO  enters 


the  water,  but,  on  entering  the  water,  it  is  bent  or  refracted 
and  takes  the  new  path  OS.  The  angle  AOR  is  called  the 

angle  of  incidence.  The  angle 
POS  is  called  the  angle  of  refrac- 
tion. 

The  angle  of  refraction  is  the 
angle  formed  by  the  refracted  ray 
and  the  perpendicular  to  the  sur- 
face at  the  point  where  the  light 
strikes  it. 

When  light  passes  from  a  rare 
the  water,  its  path  is  changed  to    medium  to  a  denser  medium,  the 

refracted   ray   is  bent  toward  the 

perpendicular,  so  that  the  angle  of  refraction  is  smaller  than 
the  angle  of  incidence.  When  a  ray  of  light  passes  from 
a  denser  to  a  rarer  medium,  the  refracted  ray  is  bent  away 
from  the  perpendicular  so  that  the  angle  of  refraction  is 
greater  than  the  angle  of  incidence. 

108.  How  Refraction  Deceives  Us.  Some  substances  re- 
fract light  more  than  others ;  for  example,  a  stick  put  in  salt 
water  seems  more  bent  than  a  stick  put  into  fresh  water. 
The  power  of  a  medium  to  bend  rays  of  light  or  deviate  them 
from  their  original  path  is  called  the  refraction  of  the  substance. 
Refraction  is  the  source  of  many  illusions ;  bent  rays  of 
light  make  objects  appear 
where  they  really  are  not. 
A  fish  at  A  (Fig.  66)  seems 
to  be  at  B.  The  end  of  the 
stick  in  Figure  64  seems 
to  be  nearer  the  surface  of 
the  water  than  it  really  is. 

The  light  from  the  sun,  moon,  and  stars  can  reach  us  only 
by  passing  through  the  atmosphere,  but  in  Section  76,  we 


FIG.  66.  —  A  fish  at  A  seems  to  be  at  B. 


USES  OF  REFRACTION  115 

learned  that  the  atmosphere  varies  in  density  from  level  to 
level;  hence  all  the  light  which  travels  through  the  atmos- 
phere is  constantly  deviated  from  its  original  path,  and  before 
the  light  reaches  the  eye  it  has  undergone  many  changes  in 
direction.  Now  we  learned  in  Section  102,  that  the  direction 
of  the  rays  of  light  as  they  enter  the  eye  determines  the  di- 
rection in  which  an  object  is  seen  ;  hence  the  sun,  moon, 
and  stars  seem  to  be  along  the  lines  which  enter  the  eye, 
although  in  reality  they  are  not. 

109.  Uses  of  Refraction.  If  it  were  not  for  refraction,  or 
the  deviation  of  light  in  its  passage  from  medium  to  medium, 
the  wonders  and  beauties  of  the  magic  lantern  and  the  cam- 
era would  be  unknown  to  us ;  sun,  moon,  and  stars  could  not 
be  made  to  yield  up  their  distant  secrets  to  us  in  photographs  ; 
the  comfort  and  help  of  spectacles  would  be  lacking,  spec- 
tacles which  have  helped  unfold  to  many  the  rare  beauties  of 
nature,  such  as  a  clear  view  of  clouds  and  sunset,  of  humming 
bee  and  flying  bird.  Books  with  their  wealth  of  entertain- 
ment and  information  would  be  sealed  to  a  large  part  of  man- 
kind, if  glasses  did  not  assist  weak  eyes. 

By  refraction  the  magnifying  glass  reveals  objects  hidden 
because  of  their  minuteness,  and  enlarges  for  our  careful  con- 
templation objects  otherwise  barely  visible.  The  watchmaker, 
unassisted  by  the  magnifying  glass,  could  not  detect  the  tiny 
grains  of  dust  or  sand  which  clog  the  delicate  wheels  of  our 
watches.  The  merchant,  with  his  lens,  examines  the  separate 
threads  of  woolen  and  silk  fabrics  to  determine  the  strength 
and  value  of  the  material.  The  physician,  with  his  invaluable 
microscope,  counts  the  number  of  infinitesimal  corpuscles  in 
the  blood  and  bases  his  prescription  on  that  count;  he  ex- 
amines the  sputum  of  a  patient  to  determine  whether  tuber- 
culosis wastes  the  system.  The  bacteriologist  with  the  same 
instrument  scrutinizes  the  drinking  water  and  learns  whether 


REFRACTION 


the  dangerous  typhoid  germs  are  present.  The  future  of 
medicine  will  depend  somewhat  upon  the  additional  secrets 
which  man  is  able  to  force  from  nature  through  the  use 
of  powerful  lenses,  because  as  lenses  have,  in  the  past, 
been  the  means  of  revealing  disease  germs,  so  in  the  fu- 
ture more  powerful  lenses  may  serve  to  bring  to  light 
germs  yet  unknown.  How  refraction  accomplishes  these 
results  will  be  explained  in  the  following  Sections. 

no.  The  Window  Pane.  We  have  seen  that  light  is  bent 
when  it  passes  from  one  medium  to  another  of  different 
density,  and  that  objects  viewed  by  refracted  light  do  not  ap- 
pear in  their  proper  positions. 

When  a  ray  of  light  passes  through  a  piece  of  plane  glass, 
such  as  a  window  pane  (Fig.  67),  it  is  refracted  at  the  point  B 

toward  the  perpendicular,  and 
continues  its  course  through 
the  glass  in  the  new  direc- 
tion BC.  On  emerging  from 
the  glass,  the  light  is  re- 
fracted away  from  the  perpen- 
dicular and  takes  the  direc- 
tion CD,  which  is  clearly  par- 
allel to  its  original  direction. 
Hence,  when  we  view  objects 
through  the  window,  we  see 

pane  seem  to  be  in  their  natural     them      slightly     displaced      in 

position,  but  otherwise  un- 
changed. The  deviation  or  displacement  caused  by  glass  as 
thin  as  window  panes  is  too  slight  to  be  noticed,  and  we  are 
not  conscious  that  objects  are  out  of  position. 

in.  Chandelier  Crystals  and  Prisms.  When  a  ray  of  light 
passes  through  plane  glass,  like  a  window  pane,  it  is  shifted 
somewhat,  but  its  direction  does  not  change ;  that  is,  the 


FIG.  67.  — Objects  looked   at  through    a 
windo 
place. 


LENSES 


117 


emergent  ray  is  parallel  to  the  incident  ray.      But  when  a 

beam  of  light  passes  through  a  triangular  glass  prism,  such 

as  a  chandelier  crystal,  its  direction  is  greatly  changed,  and 

an    object    viewed     through    a 

prism  is  seen  quite  out   of   its 

true  position. 

Whenever  lightpassesthrough 

a  prism,  it  is  bent  toward  the 

base  of  the  prism,  or  toward  the 

thick  portion  of  the  prism,  and 

emerges  from  the  prism  in  quite 

a  different  direction  from  that  in 

which  it  entered  (Fig.  68).     Hence,  when  an  object  is  looked 

at  through  a  prism,  it  is  seen  quite 
out  of  place.  In  Figure  68,  the 
candle  seems  to  be  at  S,  while  in 
reality  it  is  at  A. 

112.   Lenses.    If  two  prisms  are 
arranged  as   in  Figure   69,    and 


FIG.  68.  — When  looked  at  through 
the  prism,  A  seems  to  be  at  6". 


FIG.  69.  —  Rays  of  light   are  con- 
verged and  focused  at  f. 


two  parallel  rays  of  light  fall  upon  the  prisms,  the  beam  A 

will  be  bent  downward  toward  the  thickened  portion  of  the 

prism,  and  the  beam  B  will  be  bent 

upward  toward  the  thick  portion  of 

the  prism,  and  after  passing  through 

the  prism  the  two  rays  will  intersect 

at  some  point  F,  called  a  focus. 

If  two  prisms  are  arranged  as  in 
Figure  70,  the  ray  A  will  be  re- 
fracted upward  toward  the  thick  end,  and  the  ray  B  will  be 
refracted  downward  toward  the  thick  end ;  the  two  rays,  on 
emerging,  will  therefore  be  widely  separated  and  will  not 
intersect. 

Lenses  are  very  similar   to   prisms ;   indeed,  two  prisms 


FIG.  70.  —  Rays  of  light  are  di- 
verged and  do  not  come  to  any 
real  focus. 


REFRACTION 


123  456 

FIG.  71.  —  The  different  types  of  lenses. 


joined  as  in  Figure  69,  and  rounded  off,  would  make  a  very 

good  convex  lens.     A  lens  is  any  transparent  material,  but 

usually  glass,  with  one  or 
both  sides  curved.  The 
various  types  of  lenses 
are  shown  in  Figure  71. 

The  first  three  types 
focus  parallel  rays  at 
some  common  point  Ft  as 

in  Figure  70.     Such  lenses  are  called  convex  or  converging 

lenses.     The  last  three  types,  called  concave  lenses,  scatter 

parallel  rays  so  that  they  do  not  come  to  a  focus,   but   di- 
verge widely  after  passage  through  the  lens. 

113.   The  Shape  and  Material  of   a  Lens.     The   main  or 

principal  focus  of  a  lens,  that  is,  the  point  at   which  rays 

parallel  to  the  base  line  AB 

meet    (Fig.     71),     depends 

upon  the  shape  of  the  lens. 

For  example,  a  thick  lens, 

such  as  A  (Fig.  72),  focuses 

the  rays   very  near  to  the 

lens;    B,    which  is    not  so 

thick,  focuses  the  rays  at  a 

greater  distance   from    the 

lens ;  and  C,  which  is  a  very 

thin  lens,  focuses   the  rays 

at   a  considerable    distance 

from  the  lens.    The  distance 

Of  the  principal    foCUS   from      FIG.  72._The^0re    curved    the   lens,    the 

the  lens  is  called  the  focal 

length  of  the  lens,  and  from 

the  diagrams  we  see  that  the  more  convex  the  lens,  the  shorter 

the  focal  length. 


shorter  the  focal  length,  and  the  nearer  the 
focus  is  to  the  lens. 


HOW  LENSES  FORM  IMAGES  119 

The  position  of  the  principal  focus  depends  not  only  on  the 
shape  of  the  lens,  but  also  on  the  refractive  power  of  the 
material  composing  the  lens.  A  lens  made  of  ice  would  not 
deviate  the  rays  of  light  so  much  as  a  lens  of  similar  shape 
composed  of  glass.  The  greater  the  refractive  power  of  the 
lens,  the  greater  the  bending,  and  the  nearer  the  principal 
focus  to  the  lens. 

There  are  many  different  kinds  of  glass,  and  each  kind  of 
glass  refracts  the  light  differently.  Flint  glass  contains  lead  ; 
the  lead  makes  the  glass  dense,  and  gives  it  great  refractive 
power,  enabling  it  to  bend  and  scatter  light  in  all  directions. 
Cut  glass  and  toilet  articles  are  made  of  flint  glass  because 
of  the  brilliant  effects  caused  by  its  great  refractive  power, 
and  imitation  gems  are  commonly  nothing  more  than  polished 
flint  glass. 

114.  How  Lenses  Form  Images.  Suppose  we  place  an  arrow, 
A,  in  front  of  a  convex  lens  (Fig.  73).  The  ray  AC,  parallel 
to  the  principal  axis,  will 
pass  through  the  lens 
and  emerge  as  DE.  The 
ray  is  always  bent 
toward  the  thick  portion 
of  the  lens,  both  at  its 

FIG.  73.  —  The  image  is  larger  than  the  object. 
entrance    into    the     lens         By  means  of  a  lens,  a  watchmaker  gets  an  en- 


and  its  emergence  from 
the  lens. 

In  Section  105,  we  saw  that  two  rays  determine  the  posi- 
tion of  any  point  of  our  image;  hence  in  order  to  locate  the 
image  of  the  top  of  the  arrow,  we  need  to  consider  but  one 
more  ray  from  the  top  of  the  object.  The  most  convenient 
ray  to  choose  would  be  one  passing  through  O,  the  optical 
center  of  the  lens,  because  such  a  ray  passes  through  the  lens 
unchanged  in  direction,  as  is  clear  from  Figure  74.  The  point 


120 


REFRACTION' 


where  AC  and  AO  meet  after  refraction  will  be  the  position 
of  the  top  of  the  arrow.      Similarly  it  can  be  shown  that  the 

center  of  the  arrow  will 
be  at  the  point  T,  and 
we  see  that  the  image 
is  larger  than  the  ob- 
ject. This  can  be 
easily  proved  experi- 
mentally. Let  a  con- 
vex lens  be  placed 

FIG.  74.  — Rays  above  O  are  bent  downward,  those  near  a  Candle  (Fig.  75 )> 
below  O  are  bent  upward,  and  rays  through  O  mnvp  „  naner  srrpen 
emerge  from  the  lens  unchanged  in  direction. 

back    and     forth    be- 
hind the  lens;  for  some  position  of  the  screen  a  clear,  enlarged 

image  of  the  candle  will  be  made. 
If    the    candle    or    arrow    is 

placed  in  a  new  position,  say  at 

MA  (Fig.  76),  the  image  formed 

is  smaller  than  the  object,   and 

is  nearer  to  the    lens    than    it 

was   before.     Move  the  lens  so 

that  its  distance  from  the  candle 

is  increased,  and  then  find  the 

image  on  a  piece  of  paper.     The  size  and  position  of  the  image 

depend  upon  the  distance 
of  the  object  from  the 
lens  (Fig.  77).  By  means 
of  a  lens  one  can  easily 
get  on  a  visiting  card  a 
picture  of  a  distant  church 
steeple. 
115.  The  Value  of  Lenses.  If  it  were  not  for  the  fact  that 

a  lens  can  be  held  at  such  a  distance  from  an  object  as  to 


FIG.  75.  —  The  lens  is  held  in  such  a 
position  that  the  image  of  the  candle 
is  larger  than  the  6bject. 


A  C 

FIG.  76.  —  The  image  is  smaller  than  the  object. 


THE  HUMAN  EYE  121 

make  the  image  larger  than  the  object,  it  would  be  impossible 
for  the  lens  to  assist  the  watchmaker  in  locating  the  small 
particles  of  dust  which  clog  the 
wheels  of  the  watch.  If  it 
were  not  for  the  opposite  fact 
—  that  a  lens  can  be  held  at 
such  a  distance  from  the  object 
as  to  make  an  image  smaller  FIG.  77. -—  The  lens  is  placed  in  such  a 
than  the  object,  it  would  be  im-  fg-g  £  ™£  *  ab°"<  "» 
possible  to  have  a  photograph 

of  a  tall  tree  or  building  unless  the  photograph  were  as  large 
as  the  tree  itself.  When  a  photographer  takes  a  photograph 
of  a  person  or  a  tree,  he  moves  his  camera  until  the  image 
formed  by  the  lens  is  of  the  desired  size.  By  bringing  the 
camera  (really  the  lens  of  the  camera)  near,  we  obtain  a  large- 
sized  photograph ;  by  increasing  the  distance  between  the 
camera  and  the  object,  a  smaller  photograph  is  obtained. 
The  mountain  top  may  be  so  far  distant  that  in  the  photo- 
graph it  will  not  appear  to  be  greater  than  a  small  stone. 

Many  familiar  illustrations  of  lenses,  or  curved  refracting 
surfaces,  and  their  work,  are  known  to  all  of  us.  Fish  globes 
magnify  the  fish  that  swim  within.  Bottles  can  be  so  shaped 
that  they  make  the  olives,  pickles,  and  peaches  that  they 
contain  appear  larger  than  they  really  are.  The  fruit  in 
bottles  frequently  seems  too  large  to  have  gone  through  the 
neck  of  the  bottle.  The  deception  is  due  to  refraction,  and 
the  material  and  shape  of  the  bottle  furnish  a  sufficient  expla- 
nation. 

By  using  combinations  of  two  or  more  lenses  of  various 
kinds,  it  is  possible  to  have  an  image  of  almost  any  desired 
size,  and  in  practically  any  desired  position. 

116.  The  Human  Eye.  In  Section  114,  we  obtained  on  a 
movable  screen,  by  means  of  a  simple  lens,  an  image  of  a 


122  REFRACTION1 

candle.  The  human  eye  possesses  a  most  wonderful  lens  and 
screen  (Fig.  78);  the  lens  is  called  the  crystalline  lens,  and 

the  screen  is  called  the  retina.  Rays 
of  light  pass  from  the  object  through 
the  pupil  P,  go  through  the  crystal- 
line lens  /,,  where  they  are  refracted, 
and  then  pass  onward  to  the  retina 
Ry  where  they  form  a  distinct  image 
of  the  object.  . 

We  learned  in  Section  1 14  that  a 
change  in  the  position  of  the  object 
necessitated  a  change  in  the  position 
FIG.  78.  —  The  eye.  of  the  screen,  and  that  every  time  the 

object  was  moved  the  position  of  the 

screen  had  to  be  altered  before  a  clear  image  of  the  object 
could  be  obtained.  The  retina  of  the  eye  cannot  be  moved 
backward  and  forward,  as  the  screen  was,  and  the  crystalline 
lens  is  permanently  located  directly  back  of  the  iris.  How, 
then,  does  it  happen  that  we  can  see  clearly  both  near  and 
distant  objects;  that  the  printed  page  which  is  held  in  the 
hand  is  visible  at  one  second,  and  that  the  church  spire  on 
the  distant  horizon  is  visible  the  instant  the  eyes  are  raised 
from  the  book  ?  How  is  it  possible  to  obtain  on  an  immov- 
able screen  by  means  of  a  simple  lens  two  distinct  images  of 
objects  at  widely  varying  distances? 

The  answer  to  these  questions  is  that  the  crystalline  lens 
changes  shape  according  to  need.  The  lens  is  attached  to 
the  eye  by  means  of  small  muscles,  m,  and  it  is  by  the  action 
of  these  muscles  that  the  lens  is  able  to  become  small  arid  thick, 
or  large  and  thin;  that  is,  to  become  more  or  less  curved. 
When  we  look  at  near  objects,  the  muscles  act  in  such  a  way 
that  the  lens  bulges  out,  and  becomes  thick  in  the  middle  and 
of  the  right  curvature  to  focus  the  near  object  upon  the 


FARSIGHTEDNESS  AND  NEARSIGHTEDNESS        123 


screen.  When  we  look  at  an  object  several  hundred  feet 
away,  the  muscles  change  their  pull  on  the  lens  and  flatten  it 
until  it  is  of  the  proper  curvature  for  the  new  distance.  The 
adjustment  of  the  muscles  is  so  quick  and  unconscious  that 
we  normally  do  not  experience  any  difficulty  in  changing  our 
range  of  view  from  the  object  at  our  feet  to  the  far-distant 
hills  and  stars. 

The  ability  of  the  eye  to  adjust  itself  to  varying  distances 
is  called  accommodation.  The  power  of  adjustment  in  gen- 
eral decreases  with  age,  being  most  nearly  perfect  in  the 
young. 

117.  Farsightedness  and  Nearsightedness.  A  farsighted 
person  is  one  who  cannot  see  near  objects  so  distinctly  as  far 
objects,  and  in  many  cases  a 
farsighted  person  cannot  see 
near  objects  at  all.  The  crys- 
talline lens  is  so  thin  in  the 
center  that  even  when  the 
muscles  are  doing  their  best  to 
bulge  the  lens,  it  is  still  not 
bulged  enough  (Fig.  80)  to  focus  near  objects  upon  the  retina. 
Since  the  crystalline  lens  is  too  thin  at  the  center,  the  defect 
may  be  remedied  by  wearing  convex  lenses  which  are  thick 
enough  in  the  center  to  'balance  the  too  great  thinness  of  the 
crystalline  lens. 


FlG.  79.  —  The  normal  eye. 


FlG.  80.  —  The  farsighted  eye.    The  defect  is  remedied  by  convex  glasses. 

A  nearsighted  person  is  one  who  cannot  see  objects  dis- 
tinctly unless  they  are  very  close  to  the  eye.     In  this  case  the 


124  REFRACTION 

crystalline  lens  is  bulged  so  much  that  even  when  the  muscles 
are  doing  their  best  to  pull  it  out  from  the  center  and  flatten  it, 
the  lens  is  still  so  bulged  that  it  deviates  the  rays  too  much 
and  hence  focuses  them  before  they  reach  the  retina  (Fig.  81 ). 


FlG.  81.  —  The  nearsighted  eye.     The  defect  is  remedied  by  concave  glasses. 

Something  must  be  done  to  scatter  the  rays  so  that  they  will 
not  come  to  a  focus  before  they  reach  the  retina.  Hence 
nearsighted  persons  wear  concave  lenses  which  are  just  thin 
enough  at  the  center  to  balance  the  too  great  thickness  of  the 
crystalline  lens. 

118.  Headache  and  Eyes.  Ordinarily  the  muscles  of  ac- 
commodation adjust  themselves  easily  and  quickly;  if,  how- 
ever, they  do  not,  frequent  and  severe  headaches  occur  as  a 
result  of  too  great  muscular  effort  toward  accommodation. 
Among  young  people  headaches  are  frequently  caused  by 
over-exertion  of  the  crystalline  muscles.  Glasses  relieve  the 
muscles  of  the  extra  adjustment,  and  hence  are  effective  in 
eliminating  this  cause  of  headache. 

An  exact  balance  is  required  between  glasses,  crystalline 
lens,  and  muscular  activity,  and  only  those  who  have  studied 
the  subject  carefully  are  competent  to  treat  so  sensitive  and 
necessary  a  part  of  the  body  as  the  eye.  The  least  mistake 
in  the  curvature  of  the  glasses,  the  least  flaw  in  the  type  of 
glass  (for  example,  the  kind  of  glass  used),  means  an  im- 
proper focus,  increased  duty  for  the  muscles,  and  gradual 
weakening  of  the  entire  eye,  followed  by  headache  and 
general  physical  discomfort. 


EYE  STRAIN  125 

119.  Eye  Strain.  The  extra  work  which  is  thrown  upon 
the  nervous  system  through  seeing,  reading,  writing,  and 
sewing  with  defective  eyes  is  recognized  by  all  physicians 
as  an  important  cause  of  disease.  The  tax  made  upon  the 
nervous  system  by  the  defective  eye  lessens  the  supply  of 
energy  available  for  other  bodily  use,  and  the  general  health 
suffers.  The  health  is  improved  when  proper  glasses  are 
prescribed. 

Possibly  the  greatest  danger  of  eye  strain  is  among  school 
children,  who  are  not  experienced  enough  to  recognize  defects 
in  sight.  For  this  reason,  many  schools  employ  a  physician 
who  examines  the  pupils'  eyes  at  regular  intervals. 

The  following  general  precautions  are  worth  observing:  — 

1.  Rest  the  eyes  when  they  hurt,  and  as  far  as  possible  do 
close  work,  such  as  writing,  reading,  sewing,  wood  carving, 
etc.,  by  daylight. 

2.  Never  read  in  a  very  bright  or  a  very  dim  light. 

3.  If  the  light  is  near,  have  it  shaded. 

4.  Do  not  rub  the  eyes  with  the  fingers. 

5.  If  eyes  are  weak,  bathe  them  in  lukewarm  water  in 
which  a  pinch  of  borax  has  been  dissolved. 


CHAPTER  XII 

PHOTOGRAPHY 

120.  The  Magic  of  the  Sun.  Ribbons  and  dresses  washed 
and  hung  in  the  sun  fade ;  when  washed  and  hung  in  the 
shade,  they  are  not  so  apt  to  lose  their  color.  Clothes  are 
laid  away  in  drawers  and  hung  in  closets  not  only  for  pro- 
tection against  dust,  but  also  against  the  well-known  power 
of  light  to  weaken  color. 

Many  housewives  lower  the  window  shades  that  the  wall 
paper  may  not  lose  its  brilliancy,  that  the  beautiful  hues  of 
velvet,  satin,  and  plush  tapestry  may  not  be  marred  by  loss  in 
brilliancy  and  sheen.  Bright  carpets  and  rugs  are  sometimes 
bought  in  preference  to  more  delicately  tinted  ones,  because 
the  purchaser  knows  that  the  latter  will  fade  quickly  if  used 
in  a  sunny  room,  and  will  soon  acquire  a  dull  mellow  tone. 
The  bright  and  gay  colors  and  the  dull  and  somber  colors  are  all 
affected  by  the  sun,  but  why  one  should  be  affected  more 
than  another  we  do  not  know.  Thousands  of  brilliant  and 
dainty  hues  catch  our  eye  in  the  shop  and  on  the  street,  but 
not  one  of  them  is  absolutely  permanent ;  some  may  last  for 
years,  but  there  is  always  more  or  less  fading  in  time. 

Sunlight  causes  many  strange,  unexplained  effects.  If  the 
two  substances,  chlorine  and  hydrogen,  are  mixed  in  a  dark 
room,  nothing  remarkable  occurs  any  more  than  though  water 
and  milk  were  mixed,  but  if  a  mixture  of  these  substances  is 
exposed  to  sunlight,  a  violent  explosion  occurs  and  an  entirely 
new  substance  is  formed,  a  compound  entirely  different  in 
character  from  either  of  its  components. 

126 


THE  MAGIC   WAND  IN  PHOTOGRAPHY  127 

By  some  power  not  understood  by  man,  the  sun  is  able  to 
form  new  substances.  In  the  dark,  chlorine  and  hydrogen 
are  simply  chlorine  and  hydrogen ;  in  the  sunlight  they  com- 
bine as  if  by  magic  into  a  totally  different  substance.  By  the 
same  unexplained  power,  the  sun  frequently  does  just  the  op- 
posite work ;  instead  of  combining  two  substances  to  make 
one  new  product,  the  sun  may  separate  or  break  down  some 
particular  substance  into  its  various  elements.  For  example, 
if  the  sun's  rays  fall  upon  silver  chloride,  a  chemical  action  im- 
mediately begins,  and  as  a  result  we  have  two  separate  sub- 
stances, chlorine  and  silver.  The  sunlight  separates  silver 
chloride  into  its  constituents,  silver  and  chlorine. 

.121.  The  Magic  Wand  in  Photography.  Suppose  we  coat 
one  side  of  a  glass  plate  with  silver  chloride,  just  as  we  might 
put  a  coat  of  varnish  on  a  chair.  We  must  be  very  careful 
to  coat  the  plate  in  the  dark  room,*  otherwise  the  sunlight  will 
separate  the  silver  chloride  and  spoil  our  plan.  Then  lay  a 
horseshoe  on  the  plate  for  good  luck,  and  carry  the  plate  out 
into  the  light  for  a  second.  The  light  will  separate  the  silver 
chloride  into  chlorine  and  silver,  the  latter  of  which  will  re- 
main on  the  plate  as  a  thin  film.  All  of  the  plate  was  affected 
by  the  sun  except  the  portion  protected  by  the  horseshoe  which, 
because  it  is  opaque,  would  not  allow  light  to  pass  through 
and  reach  the  plate.  If  now  the  plate  is  carried  back  to  the 
dark  room  and  the  horseshoe  is  removed,  one  would  expect  to 
see  on  the  plate  an  impression  of  the  horseshoe,  because  the 
portion  protected  by  the  horseshoe  would  be  covered  by 
silver  chloride  and  the  exposed  unprotected  portion  would  be 
covered  by  metallic  silver.  But  we  are  much  disappointed 
because  the  plate,  when  examined  ever  so  carefully,  shows 
not  the  slightest  change  in  appearance.  The  change  is  there, 
but  the  unaided  eye  cannot  detect  the  change.  Some  chemi- 

*  That  is,  a  room  from  which  ordinary  daylight  is  excluded. 


128 


PHOTOGRAPHY 


cal,  the  so-called  "developer,"  must  be  used  to  bring  out  the 
hidden  change  and  to  reveal  the  image  to  our  unseeing  eyes. 
There  are  many  different  developers  in  use,  any  one  of  which 
will  effect  the  necessary  transformation.  When  the  plate  has 
been  in  the  developer  for  a  few  seconds,  the  silver  coating 
gradually  darkens,  and  slowly  but  surely  the  image  printed  by 
the  sun's  rays  appears.  But  we  must  not  take  this  picture 
into  the  light,  because  the  silver  chloride  which  was  protected 
by  the  horseshoe  is  still  present,  and  would  be  strongly 
affected  by  the  first  glimmer  of  light,  and,  as  a  result,  our  en- 
tire plate  would  become  similar  in  character  and  there  would 
be  no  contrast  to  give  an  image  of  the  horseshoe  on  the  plate. 
But  a  photograph  on  glass,  which  must  be  carefully  shielded 
from  the  light  and  admired  only  in  the  dark  room,  would  be 
neither  pleasurable  nor  practical.  If  there  were  some  way  by 
which  the  hitherto  unaffected  silver  chloride  could  be  totally 
removed,  it  would  be  possible  to  take  the  plate  into  any  light 
without  fear.  To  accomplish  this,  the  unchanged  silver 
chloride  is  got  rid  of  by  the  process  technically  called  "  fix- 
ing"; that  is,  by  washing  off  the  unreduced  silver  chloride 

with  a  solution  such 
as  sodium  thiosulphite, 
commonly  known  as 
hypo.  After  a  bath  in 
the  hypo  the  plate  is 
cleansed  in  clear  run- 
ning water  and  left  to 
dry.  Such  a  process 
gives  a  clear  and  perma- 
nent picture  on  the  plate. 
122.  The  Camera.  A 

camera  (Fig.  82)  is   a  light-tight  box  containing  a  movable 
convex  lens  at  one  end  and  a  screen  at  the  opposite   end. 


FIG.  82.  —  A  camera. 


LIGHT  AND  SHADE  1 29 

Light  from  the  object  to  be  photographed  passes  through 
the  lens,  falls  upon  the  screen,  and  forms  an  image  there. 
If  we  substitute  for  the  ordinary  screen  a  plate  or  film 
coated  with  silver  chloride  or  any  other  silver  salt,  the 
light  which  falls  upon  the  sensitive  plate  and  forms  an  image 
there  will  change  the  silver  chloride  and  produce  a  hidden 
image.  If  the  plate  is  then  removed  from  the  camera  in  the 
dark,  and  is  treated  as  described  in  the  preceding  Section,  the 
image  becomes  visible  and  permanent.  In  practice  some 
gelatin  is  mixed  with  the  silver  salt,  and  the  mixture  is  then 
poured  over  the  plate  or  film  in.  such  a  way  that  a  thin,  even 
coating  is  made.  It  is  the  presence  of  the  gelatin  that  gives 
plates  a  yellowish  hue.  The  sensitive  plates  are  left  to  dry 
in  dark  rooms,  and  when  the  coating  has  become  absolutely 
firm  and  dry,  the  plates  are  packed  in  boxes  and  sent  forth 
for  sale. 

Glass  plates  are  heavy  and  inconvenient  to  carry,  so  that 
celluloid  films  have  almost  entirely  taken  their  place,  at  least 
for  outdoor  work. 

123.  Light  and  Shade.  Let  us  apply  the  above  process  to 
a  real  photograph.  Suppose  we  wish  to  take  the  photograph 
of  a  man  under  the  historic  trees  of  Mt.  Vernon.  If  the  man 
wore  gray  trousers,  a  black  coat,  and  white  collar,  these  details 
must  be  faithfully  represented  in  the  photograph.  How  can 
the  almost  innumerable  lights  and  shades  be  produced  on  the 
plate  ? 

The  white  collar  would  send  through  the  lens  the  most  light 
to  the  sensitive  plate  ;  hence  the  silver  chloride  on  the  plate  would 
be  most  changed  at  the  place  where  the  lens  formed  an  image 
of  the  collar.  Gray  trousers  would  not  send  to  the  lens  so  much 
light  as  the  white  collar,  hence  the  silver  chloride  would  be  less 
affected  by  the  light  from  the  trousers  than  by  that  from  the 
collar,  and  at  the  place  where  the  lens  produced  an  image  of 

CL.    GEN.    SCJ.  —  Q 


130 


PHOTOGRAPHY 


the  trousers  the  silver  chloride  would  not  be  changed  so  much 
as  where  the  collar  image  is.  The  light  from  the  face  would 
produce  a  still  different  effect,  since  the  light  from  the  face  is 
stronger  than  the  light  from  the  gray  trousers,  but  less  than 
that  from  a  white  collar.  The  face  in  the  image  would  show 
less  changed  silver  chloride  than  the  collar,  but  more  than  the 
trousers,  because  the  face  is  lighter  than  the  trousers,  but  not 
so  light  as  the  collar.  Finally,  the  silver  chloride  would  be  least 
affected  by  the  dark  coat.  The  trees  in  the  background  would 
affect  the  plate  according  to  the  brightness  of  the  light  which 
fell  directly  upon  them  and  which  they  reflected  to  the  camera. 
When  such  a  plate  has  been  developed  and  fixed,  as  described 
in  Section  12-1,  we  have  the  so-called  negative  (Fig.  83).  The 

collar  is  very  dark,  the  black 
coat  white,  the  clear  blue  sky 
very  dark. 

.The  lighter  the  object,  such 
as  sky  or  collar,  the  more  salt 
is  changed,  or,  in  other  words, 
the  greater  the  portion  of  the 
silver  salt  that  is  affected,  and 
hence  the  darker  the  stain  on 
the  plate  at  that  particular 
spot.  The  plate  shows  all  gra- 
dations of  intensity  —  the  sky  is 
dark,  the  black  coat  is  light. 
The  photograph  is  true  as  far 
as  position,  form,  and  expression  are  concerned,  but  the  actual 
intensities  are  just  reversed.  How  this  plate  can  be  trans- 
formed into  a  photograph  true  in  every  detail  will  be  seen  in 
the  following  Section. 

124.     The    Perfect  Photograph.     Bright   objects,    such  as 
the  sky  or  a  white  waist,  change  much  of  the  silver  chlo- 


FlG.  83.  —  A  negative. 


LIGHT  AND  DISEASE  131 

ride,  and  hence  appear  dark  on  the  negative.  Dark  objects, 
such  as  furniture  or  a  black  coat,  change  little  of  the  chlo- 
ride, and  hence  appear  light  on  the  negative.  To  obtain  a 
true  photograph,  the  negative  is  placed  on  a  piece  of  sen- 
sitive photographic  paper,  or  paper  coated  with  a  silver  salt 
in  the  same  manner  as  the  plate  and  films.  The  combination 
is  exposed  to  the  light.  The  dark  portions  of  the  negative 
will  act  as  obstructions  to  the  passage  of  light,  and  but  little 
light  will  pass  through  that  part  of  the  negative  to  the  photo- 
graphic paper,  and  consequently  but  little  of  the  silver  salt 
on  the  paper  will  be  changed.  On  the  other  hand,  the  light 
portion  of  the  negative  will  allow  free  and  easy  passage  of 
the  light  rays,  which  will  fall  upon  the  photographic  paper  and 
will  change  much  more  of  the 
silver.  Thus  it  is  that  dark 
places  in  the  negative  produce 
light  places  in  the  positive 
or  real  photograph  (Fig.  84), 
and  that  light  places  in  the 
negative  produce  dark  places 
in  the  positive ;  all  intermediate 
grades  are  likewise  represented 
with  their  proper  gradations  of 
intensity. 

If  properly  treated,  a  nega- 
tive remains  good   for   years, 

.        .„  .  ..     ,     _     .  FIG.  84.  —  A  positive  or  true  photograph. 

and  will  serve  for  an  indefinite 

number  of  positives  or  true  photographs. 

125.  Light  and  Disease.  The  far-reaching  effect  which 
light  has  upon  some  inanimate  objects,  such  as  photographic 
films  and  clothes,  leads  us  to  inquire  into  the  relation  which 
exists  between  light  and  living  things.  We  know  from  daily 
observation  that  plants  must  have  light  in  order  to  thrive 


132 


PHOTOGRAPHY 


and  grow.  A  healthy  plant  brought  into  a  dark  room  soon 
loses  its  vigor  and  freshness,  and  becomes  yellow  and  droop- 
ing. Plants  do  not  all 
agree  as  to  the  amount 
of  light  they  require, 
for  some,  like  the  violet 
and  the  arbutus,  grow 
best  in  moderate  light, 
while  others,  like  the 
willows,  need  the 
strong,  full  beams  of 

FIG.  85.  — Stems  and  leaves  of  oxalis  growing          the  SUn.      But  nearly  all 

common  plants,  what- 
ever they  are,  sicken  and  die  if  deprived  of  sunlight  for  a  long 
time.  This  is  likewise  true  in  the  animal  world.  During  long 
transportation,  animals  are  sometimes  necessarily  confined  in 
dark  cars,  with  the  result  that  many  deaths  occur,  even  though 
the  car  is  well  aired  and  ventilated  and  the  food  supply 
good.  Light  and  fresh  air  put  color  into  pale  cheeks,  just  as 
light  and  air  transform  sickly,  yellowish  plants  into  hardy 
green  ones.  Plenty  of  fresh  air,  light,  and  pure  water  are 
the  watchwords  against  disease. 

In  addition  to  the  plants  and  animals  which  we  see,  there 
are  many  strange  unseen  ones  floating  in  the  atmosphere 
around  us,  lying  in  the  dust  of  corner  and  closet,  growing 
in  the  water  we  drink,  and  thronging  decayed  vegetable 
and  animal  matter.  Every  one  knows  that  mildew  and  ver- 
min do  damage  in  the  home  afid  in  the  field,  but  very  few 
understand  that,  in  addition  to  these  visible  enemies  of 
man,  there  are  swarms  of  invisible  plants  and  animals  some 
of  which  do  far  more  damage,  both  directly  and  indirectly, 
than  the  seen  and  familiar  enemies.  All  such  very  small 
plants  and  animals  are  known  as  microorganisms. 


LIGHT  AND  DISEASE  133 

Not  all  microorganisms  are  harmful ;  some  are  our  friends 
and  are  as  helpful  to  us  as  are  cultivated  plants  and  domesti- 
cated animals.  Among  the  most  important  of  the  microor- 
ganisms are  bacteria,  which  include  among  their  number  both 
friend  and'  foe.  In  the  household,  bacteria  are  a  fruitful 
source  of  trouble,  but  some  of  them  are  distinctly  friends. 
The  delicate  flavor  of  butter  and  the  sharp  but  pleasing  taste 
of  cheese  are  produced  by  bacteria.  On  the  other  hand, 
bacteria  are  the  cause  of  many  of  the  most  dangerous  diseases, 
such  as  typhoid  fever,  tuberculosis,  influenza,  and  la  grippe. 

By  careful  observation  and  experimentation  it  has  been 
shown  conclusively  that  sunlight  rapidly  kills  bacteria,  and 
that  it  is  only  in  dampness  and  darkness  that  bacteria  thrive 
and  multiply.  Although  sunlight  is  essential  to  the  growth 
of  most  plants  and  animals,  it  retards  and  prevents  the  growth 
of  bacteria.  Dirt  and  dust  exposed  to  the  sunlight  lose  their 
living  bacteria,  while  in  damp  cellars  and  dark  corners  the 
bacteria  thrive,  increasing  steadily  in  number.  For  this  rea- 
son our  houses  should  be  kept  light  and  airy ;  blinds  should 
be  raised,  even  if  carpets  do  fade  ;  it  is  better  that  carpets  and 
furniture  should  fade  than  that  disease-producing  bacteria 
should  find  a  permanent  abode  within  our  dwellings.  Kitch- 
ens and  pantries  in  particular  should  be  thoroughly  lighted. 
Bedclothes,  rugs,  and  clothing  should  be  exposed  to  the  sun- 
light as  frequently  as  possible ;  there  is  no  better  safeguard 
against  bacterial  disease  than  light.  In  a  sick  room  sunlight 
is  especially  valuable,  because  it  not  only  kills  bacteria,  but 
keeps  the  air  dry,  and  new  bacteria  cannot  get  a  start  in  a 
dry  atmosphere. 


CHAPTER    XIII 


COLOR 

126.  The  Rainbow.     One  of  the  most  beautiful  and  well- 
known  phenomena  in  nature  is  the  rainbow,  and  from  time 
immemorial  it  has  been  considered  Jehovah's  signal  to  man- 
kind that  the  storm  is  over  and  that  the  sunshine  will  remain. 
Practically  every  one  knows  that  a  rainbow  can  be  seen  only 
when  the  sun's  rays  shine  upon  a  mist  of  tiny  drops  of  water. 
It  is  these  tiny  drops  which  by  their  refraction  and  their  scat- 
tering of  light  produce  the  rainbow  in  the  heavens. 

The  exquisite  tints  of  the  rainbow  can  be  seen  if  we  look 
at  an  object  through  a  prism  or  chandelier  crystal,  and  a  very 
simple  experiment  enables  us  to  produce  on  the  wall  of  a 
room  the  exact  colors  of  the  rainbow  in  all  their  beauty. 

127.  How   to   produce    Rainbow    Colors.       The    Spectrum. 
If  a  beam  of  sunlight  is  admitted  into  a  dark  room  through 
a  narrow  opening  in  the  shade,  and  is  allowed  to  fall  upon 

a  prism,  as  shown  in 
Figure  86,  a  beautiful 
band  of  colors  will  appear 
on  the  opposite  wall  of 
the  room.  The  ray  of 
light  which  entered  the 
room  as  ordinary  sun- 
light has  not  only  been 
refracted  and  bent  from  its  straight  path,  but  it  has  been 
spread  out  into  a  band  of  colors  similar  to  those  of  the  rainbow. 

134 


FIG.  86.  —  White  light  is  a  mixture  of  lights  of 
rainbow  colors. 


COLOR 


135 


Whenever  light  passes  through  a  prism  or  lens,  it  is  dis- 
persed or  separated  into  all  the  colors  which  it  contains,  and 
a  band  of  colors  produced  in  this  way  is  called  a  spectrum. 
If  we  examine  such  a  spectrum  we  find  the  following  colors 
in  order,  each  color  imperceptibly  fading  into  the  next  :  violet, 
indigo,  blue,  green,  yellow,  orange,  red. 

128.  Sunlight  or  White  Light.    White  light  or  sunlight  can 
be  dispersed  or  separated  into  the  seven  primary  colors  or 
rainbow    hues,   as   shown    in   the  preceding  Section.     What 
seems  even  more  wonderful  is  that  the  seven  spectral  colors 
can  be  recombined  so  as  to  make  white  light. 

If  a  prism  B  (Fig.  87)  exactly  similar  to  A  in  every  way  is 
placed  behind  A  in  a.  reversed  position,  it  will  undo  the 
dispersion  of  A,  bending 
upward  the  seven  different 
beams  in  such  a  way  that 
they  emerge  together  and 
produce  a  white  spot  on  the 
screen.  Thus  we  see,  from  ^ 
two  simple  experiments,  that 
all  the  colors  of  the  rainbow 
may  be  obtained  from  white 

light,  and  that  these  colors  may  be  in  turn  recombined  to 
produce  white  light. 

White  light  is  not  a  simple  light,  but  is  composed  of  all  the 
colors  which  appear  in  the  rainbow. 

129.  Color.     If  a  piece  of  red  glass  is  held  in  the  path  of 
the  colored  beam  of  light  formed  as  in  Section  127,  all  the 
colors  on  the  wall  will  disappear  except  the  red,  and  instead 
of  a  beautiful  spectrum  of  seven  colors  there  will  be  seen  the 
red  color  alone.     The  red  glass  does  not  allow  the  passage 
through  it  of  any  light  except  red  light  ;  all  other  colors  are 
absorbed  by  the  red  glass  and  do  not  reach  the  eye.     Only 


136  COLOR 

the  red  ray  passes  through  the  red  glass,  reaches  the  eye, 
and  produces  a  sensation  of  color. 

If  a  piece  of  blue  glass  is  substituted  for  the  red  glass,  the 
blue  band  remains  on  the  wall,  while  all  the  other  colors  dis- 
appear. If  both  blue  and  red  pieces  of  glass  are  held  in  the 
path  of  the  beam,  so  that  the  light  must  pass  through  first 
one  and  then  the  other,  the  entire  spectrum  disappears  and 
no  color  remains.  The  blue  glass  absorbs  all  of  the  rays 
except  the  blue  ones,  and  the  red  glass  will  not  allow  these 
blue  rays  to  pass  through  it ;  hence  no  light  is  allowed  pas- 
sage to  the  eye. 

An  emerald  looks  green  because  it  transmits  green,  and 
absorbs  all  the  other  six  colors  of  which  ordinary  daylight  is 
composed.  A  diamond  appears  white  because  it  allows  the 
passage  through  it  of  all  the  various  rays ;  this  is  likewise 
true  of  water  and  window  panes. 

Stained-glass  windows  owe  their  charm  and  beauty  to  the 
presence  in  the  glass  of  various  dyes  and  pigments  which 
absorb  in  different  amounts  some  colors  from  white  light  and 
transmit  others.  These  pigments  or  dyes  are  added  to  the 
glass  while  it  is  in  the  molten  state,  and  the  beauty  of  a 
stained-glass  window  depends  largely  upon  the  richness  and 
the  delicacy  of  the  pigments  used. 

130.  Reflected  Light.  Opaque  Objects.  In  Section  106 
we  learned  that  most  objects  are  visible  to  us  because  of  the 
light  diffusely  reflected  from  them.  A  white  object,  such  as 
a  sheet  of  paper,  a  whitewashed  fence,  or  a  table  cloth,  absorbs 
little  of  the  light  which  falls  upon  it,  but  reflects  nearly  all, 
thus  producing  the  sensation  of  white.  A  red  carpet  absorbs 
all  the  light  rays  incident  upon  it  except  the  red  rays,  and 
these  it  reflects  to  the  eye. 

Any  substance  or  object  which  reflects  none  of  the  rays 
which  fall  upon  it,  but  absorbs  all,  appears  black ;  no  rays 


HOW  AND    WHY  COLORS   CHANGE  137 

reach  the  eye,  and  there  is  an  absence  of  any  color  sensation. 
Coal  and  tar  and  soot  are  good  illustrations  of  objects  which 
absorb  all  the  light  which  falls  upon  them. 

131.  How  and  Why  Colors  Change.  Matching  Colors. 
Most  women  prefer  to  shop  in  the  morning  and  early  after- 
noon when  the  sunlight  illuminates  shops  and  factories,  and 
when  gas  and  electricity  do  not  throw  their  spell  over  colors. 
Practically  all  people  know  that  ribbons  and  ties,  trimmings 
and  dresses,  frequently  look  different  at  night  from  what  they 
do  in  the  daytime.  It  is  not  safe  to  match  colors  by  artificial 
light ;  cloth  which  looks  red  by  night  may  be  almost  purple 
by  day.  Indeed,  the  color  of  an  object  depends  upon  the 
color  of  the  light  which  falls  upon  it.  Strange  sights  are 
seen  on  the  Fourth  of  July  when  variously  colored  fireworks 
are  blazing.  The  child  with  a  white  blouse  appears  first  red, 
then  blue,  then  green,  according  as  his  powders  burn  red,  blue, 
or  green.  The  face  of  the  child  changes  from  its  normal 
healthy  hue  to  a  brilliant  red  and  then  to  ghastly  shades. 

Suppose,  for  example,  that  a  white  hat  is  held  at  the  red 
end  of  the  spectrum  or  in  any  red  light.  The  characteristics 
of  white  objects  is  their  ability  to  reflect  all  the  vario.us  rays 
that  fall  upon  them.  Here,  however,  the  only  light  which 
falls  upon  the  white  hat  is  red  light,  hence  the  only  light 
which  the  hat  has  to  reflect  is  red  light  and  the  hat  conse- 
quently appears  red.  Similarly,  if  a  white  hat  is  placed  in  a 
blue  light,  it  will  reflect  all  the  light  which  falls  upon  it,  namely, 
blue  light,  and  will  appear  blue.  If  a  red  hat  is  held  in  a  red 
light,  it  is  seen  in  its  proper  color.  If  a  red  hat  is  held  in  a  blue 
light,  it  appears  black  ;  it  cannot  reflect  any  of  the  blue  light 
because  that  is  all  absorbed  and  there  is  no  red  light  to  reflect. 

A  child  wearing  a  green  frock  on  Independence  Day  seems 
at  night  to  be  wearing  a  black  frock,  if  standing  near  powders 
burning  with  red,  blue,  or  violet  light. 


138 


COLOR 


132.    Pure,  Simple  Colors  —  Things  as  they  Seem.     To  the 

eye  white  light  appears  a  simple,  single  color.  It  reveals  its 
compound  nature  to  us  only  when  passed  through  a  prism, 
when  it  shows  itself  to  be  compounded  of  an  infinite  number 
of  colors  which  Sir  Isaac  Newton  grouped  in  seven  divisions  : 
violet,  indigo,  blue,  green,  yellow,  orange,  and  red. 

We  naturally  ask  ourselves  whether  these  colors  which 
compose  white  light  are  themselves  in  turn  compound  ?  To 
answer  that  question,  let  us  very  carefully  insert  a  second 
prism  in  the  path  of  the  rays  which  issue  from  the  first  prism, 
carefully  barring  out  the  remaining  six  kinds  of  rays.  If  the 
red  light  is  compound,  it  will  be  broken  up  into  its  constit- 
uent parts  and  will  form  a  typical  spectrum  of  its  own,  just  as 
white  light  did  after  its  passage  through  a  prism.  But  the  red 
rays  pass  through  the  second  prism,  are  refracted,  and  bent 
from  this  course,  and  no  new  colors  appear,  no  new  spectrum 
is  formed.  Evidently  a  ray  of  spectrum  red  is  a  simple 

color,  not  a  compound 
color. 

If  a  similar  experi- 
ment is  made  with  the 
remaining  spectrum 
rays,  the  result  is 
always  the  same :  the 
individual  spectrum 
colors  remain  simple, 

Green,  blue,     pure    Colors.      T '//  e 

individual     spectrum 


FlG.  88.  —  Violet  and  green  give  blue, 
and  red  give  white. 


colors  are  groups  of  simple,  pure  colors. 

133.  Colors  not  as  they  Seem  —  Compound  Colors.  If  one 
half  of  a  cardboard  disk  (Fig.  88)  is  painted  green,  and  the  other 
half  violet,  and  the  disk  is  slipped  upon  a  toy  top,  and  spun  rap- 
idly, the  rotating  disk  will  appear  blue ;  if  red  and  green  are 


THE  ESSENTIAL   COLORS  139 

used  in  the  same  way  instead  of  green  and  violet,  the  rotating 
disk  will  appear  yellow.  A  combination  of  red  and  yellow 
will  give  orange.  The  colors  formed  in  this  way  do  not  appear 
to  the  eye  different  from  the  spectrum  colors,  but  they  are 
actually  very  different.  The  spectrum  colors,  as  we  saw  in 
the  preceding  Section,  are  pure,  simple  colors,  while  the  colors 
formed  from  the  rotating  disk  are  in  reality  compounded  of 
several  totally  different  rays,  although  in  appearance  the 
resulting  colors  are  pure  and  simple. 

If  it  were  not  that  colors  can  be  compounded,  we  should 
be  limited  in  hue  and  shade  to  the  seven  spectral  colors ;  the 
wealth  and  beauty  of  color  in  nature,  art,  and  commerce  would 
be  unknown ;  the  flowers  with  their  thousands  of  hues  would 
have  a  poverty  of  color  undreamed  of;  art  would  lose  its 
magenta,  its  lilac,  its  olive,  its  lavender,  and  would  have  to 
work  its  wonders  with  the  spectral  colors  alone.  By  com- 
pounding various  colors  in  different  proportions,  new  colors 
can  be  formed  to  give  freshness  and  variety.  If  one  third  of 
the  rotating  disk  is  painted  blue,  and  the  remainder  white,  the 
result  is  lavender ;  if  fifteen  parts  of  white,  four  parts  of  red, 
and  one  part  of  blue  are  arranged  on  the  disk,  the  result  is 
lilac.  Olive  is  obtained  from  a  combination  of  two  parts  green, 
one  part  red,  and  one  part  black ;  and  the  soft  rich  shades  of 
brown  are  all  due  to  different  mixtures  of  black,  red,  orange, 
or  yellow. 

134.  The  Essential  Colors.  Strange  and  unexpected  facts 
await  us  at  every  turn  in  science !  If  the  rotating  cardboard 
disk  (Fig.  88)  is  painted  one  third  red,  one  third  green,  and 
one  third  blue,  the  resulting  color  is  white.  While  the  mix- 
ture of  the  seven  spectral  colors  produces  white,  it  is  not 
necessary  to  have  all  of  those  seven  colors  in  order  to  obtain 
white ;  because  a  mixture  of  the  following  colors  alone,  red, 
green,  and  blue,  will  give  white.  Moreover,  by  the  mixture  of 


140  COLOR 

these  three  colors  in  proper  proportions,  any  color  of  the 
spectrum,  such  as  yellow  or  indigo  or  orange,  may  be  ob- 
tained. The  three  spectral  colors,  red,  green,  and  blue,  are 
called  primary  or  essential  hues,  because  all  known  tints  of 
color  may  be  produced  by  the  careful  blending  of  blue,  green, 
and  red  in  the  proper  proportions ;  for  example,  purple  is 
obtained  by  the  blending  of  red  and  blue,  and  orange  by  the 
blending  of  red  and  yellow. 

135-  Color  Blindness.  The  nerve  fibers  of  the  eye  which 
carry  the  sensation  of  color  to  the  brain  are  particularly  sensi- 
tive to  the  primary  colors  —  red,  green,  blue.  Indeed,  all 
color  sensations  are  produced  by  the  stimulation  of  three  sets 
of  nerves  which  are  sensitive  to  the  primary  colors.  If  one 
sees  purple,  it  is  because  the  optic  nerves  sensitive  to  red 
and  blue  (purple  equals  red  plus  blue)  have  carried  their  sep- 
arate messages  to  the  brain,  and  the  blending  of  the  two  dis- 
tinct messages  in  the  brain  has  given  the  sensation  of  purple. 
If  a  red  rose  is  seen,  it  is  because  the  optic  nerves  sensitive 
to  red  have  been  stimulated  and  have  carried  the  message  to 
the  brain. 

A  snowy  field  stimulates  equally  all  three  sets  of  optic 
nerves  —  the  red,  the  green,  and  the  blue.  Lavender,  which 
is  one  part  blue  and  three  parts  white,  would  stimulate  all 
three  sets  of  nerves,  but  with  a  minimum  of  stimulation  for 
the  blue.  Equal  stimulation  of  the  three  sets  would  give  the 
impression  of  white. 

A  color-blind  person  has  some  defect  in  one  or  more  of  the 
three  sets  of  nerves  which  carry  the  color  message  to  the  brain. 
Suppose  the  nerve  fibers  responsible  for  carrying  the  red  are 
totally  defective.  If  the  person  views  a  yellow  flower,  he  will 
see  it  as  a  green  flower.  Yellow  is  made  up  of  red  and  green, 
and  hence  both  the  red  and  green  nerve  fibers  should  be 
stimulated,  but  the  red  nerve  fibers  are  defective  and  do  not 


COLOR  BLINDNESS  141 

respond,  the  green  nerve  fibers  alone  being  stimulated,  and 
the  brain  therefore  interprets  green. 

A  well-known  author  gives  an  amusing  incident  of  a  dinner 
party,  at  which  the  host  offered  stewed  tomato  for  apple 
sauce.  What  color  nerves  were  defective  in  the  case  of  the 
host  ? 

In  some  employments  color  blindness  in  an  employee  would 
be  fatal  to  many  lives.  Engineers  and  pilots  govern  the  di- 
rection and  speed  of  trains  and  boats  largely  by  the  colored 
signals  which  flash  out  in  the  night's  darkness  or  move  in  the 
day's  bright  light,  and  any  mistake  in  the  reading  of  color 
signals  would  imperil  the  lives  of  travelers.  For  this  reason 
a  rigid  test  in  color  is  given  to  all  persons  seeking  such  em- 
ployment, and  the  ability  to  match  ribbons  and  yarns  of  all 
ordinary  hues  is  an  unvarying  requirement  for  efficiency. 


CHAPTER  XIV 

HEAT  AND  LIGHT  AS  COMPANIONS 

<JThe  night  has  a  thousand  eyes, 
~  And  the  day  but  one^. 
Yet  the  light  of  the  bright  world  dies 
With  the  dying  sun." 

136.  Most  bodies  which  glow  and  give  out  light  are  hot ;  the 
stove  which  glows  with  a  warm  red  is  hot  and  fiery ;  smolder- 
ing wood  is  black  and  lifeless;  glowing  coals  are  far  hotter 
than  black  ones.     The  stained-glass  window  softens  and  mel- 
lows the  bright  light  of  the  sun,  but  it  also  shuts  out  some 
of  the  warmth  of  the  sun's  rays ;  the  shady  side  of  the  street 
spares  our  eyes  the  intense  glare  of  the  sun,  but  may  chill  us 
by  the  absence  of  heat.     Our  illumination,  whether  it  be  oil 
lamp  or  gas  jet  or  electric  light,  carries  with  it  heat ;  indeed,, 
so  much  heat  that  we  refrain  from  making  a  light  on  a  warm 
summer's  night  because  of  the  heat  which  it  unavoidably  fur- 
nishes. 

137.  Red  a  Warm  Color.     We   have  seen  that  heat  and 
light  usually  go  hand  in  hand.     In  summer  we  lower  the 
shades  and  close  the  blinds  in  order  to  keep  the  house  cool, 
because  the  exclusion  of  light  means  the  exclusion  of  some 
heat ;  in  winter  we  open  the  blinds  and  raise  the  shades  in 
order  that  the  sun  may  stream  into  the  room  and  flood  it  with 
light  and  warmth.     The  heat  of  the  sun  and  the  light  of  the 
sun  seem  boon  companions. 

We  can  show  that  when  light  passes  through  a  prism  and 
is  refracted,  forming  a  spectrum,  as  in  Section  127,  it  is  accom- 

142 


THE  ENERGY  OF  THE  SUN 


143 


panied  by  heat.  If  we  hold  a  sensitive  thermometer  in  the 
violet  end  of  the  spectrum  so  that  the  violet  rays  fall  upon  the 
bulb,  the  reading  of  the  mercury  will  be  practically  the  same  as 
when  the  thermometer  is  held  in  any  dark  part  of  the  room  ;  if, 
however,  the  thermometer  is  slowly  moved  toward  the  red  end 
of  the  spectrum,  a  change  occurs  and  the  mercury  rises  slowly 
but  steadily,  showing  that  heat  rays  are  present  at  the  red 
end  of  the  spectrum.  This  agrees  with  the  popular  notion, 
formed  independently  of  science,  which  calls  the  reds  the 
warm  colors.  Every  one  of  us  associates  red  with  warmth ; 
in  the  summer  red  is  rarely  worn,  it  looks  hot;  but  in  winter 
red  is  one  of  the  most  pleasing  colors  because  of  the  sense  of 
warmth  and  cheer  it  brings. 

All  light  rays  are  accompanied  by  a  small  amount  of  heat ', 
but  tJie  red  rays  carry  the  most. 

What  seems  perhaps  the  most  unexpected  thing,  is  that 
the  temperature,  as  indicated  by  a  sen- 
sitive thermometer,  continues  to  rise  if 
the  thermometer  is  moved  just  beyond 
the  red  light  of  the  spectrum.  There 
actually  seems  to  be  more  heat  beyond 
the  red  than  in  the  red,  but  if  the 
thermometer  is  moved  too  far  away, 
the  temperature  again  falls.  Later 
we  shall  see  what  this  means. 

138.  The  Energy  of  the  Sun.  It 
is  difficult  to  tell  how  much  of  the 
energy  of  the  sun  is  light  and  how 

much  is  heat,  but  it  is  easy  to  deter-   FlG  89._The  energy  of  the 
mine  the  combined  effect  of  heat  and 
light. 

Suppose  we  allow  the  sun's  rays  to  fall   perpendicularly 
upon  a  metal  cylinder  coated  with  lampblack  and  filled  with 


sun  can  be  measured  in  heat 
units. 


144  HEAT  AND   LIGH7"  AS   COMPANIONS 

a  known  quantity  of  water  (Fig.  89);  at  the  expiration  of  a 
few  hours  the  temperature  of  the  water  will  be  considerably 
higher.  Lampblack  is  a  good  absorber  of  heat,  and  it  is  used 
as  a  coating  in  order  that  all  the  light  rays  which  fall  upon 
the  cylinder  may  be  absorbed  and  none  lost  by  reflection. 

Light  and  heat  rays  fall  upon  the  lampblack,  pass  through 
the  cylinder,  and  heat  the  water.  We  know  that  the  red 
light  rays  have  the  largest  share  toward  heating  the  water, 
because  if  the  cylinder  is  surrounded  by  blue  glass  which  ab- 
sorbs the  red  rays  and  prevents  their  passage  into  the  water, 
the  temperature  of  the  water  begins  to  fall.  That  the  other 
light  rays  have  a  small  share  would  have  been  clear  from  the 
preceding  'Section. 

All  the  energy  of  the  sunshine  which  falls  upon  the  cylin- 
der, both  as  heat  and  as  light,  is  absorbed  in  the  form  of  heat, 
and  the  total  amount  of  this  energy  can  be  calculated  from 
the  increase  in  the  temperature  of  the  water.  The  energy 
which  heated  the  water  would  have  passed  onward  to  the  sur- 
face of  the  earth  if  its  path  had  not  been  obstructed  by  the 
cylinder  of  water ;  and  we  can  be  sure  that  the  energy  which 
entered  the  water  and  changed  its  temperature  would  ordi- 
narily have  heated  an  equal  area  of  the  earth's  surface ;  and 
from  this,  we  can  calculate  the  energy  falling  upon  the  entire 
surface  of  the  earth  during  any  one  day. 

Computations  based  upon  this  experiment  show  that 
the  earth  receives  daily  from  the  sun  the  equivalent  of 
341,000,000,000  horse  power — an  amount  inconceivable 
to  the  human  mind. 

Professor  Young  gives  a  striking  picture  of  what  this 
energy  of  the  sun  could  do.  A  solid  column  of  ice  93,000,000 
miles  long  and  2\  miles  in  diameter  could  be  melted  in  a 
single  second  if  the  sun  could  concentrate  its  entire  power 
on  the  ice. 


HOW  LIGHT  AND   HEAT  TRAVEL 


145 


While  the  amount  of  energy  received  daily  from  the  sun 
by  the  earth  is  actually  enormous,  it  is  small  in  comparison 
with  the  whole  amount  given  out  by  the  sun  to  the  numerous 
heavenly  bodies  which  make  up  the  universe.  In  fact,  of  the 
entire  outflow  of  heat  and  light,  the  earth  receives  only  one 
part  in  two  thousand  million,  and  this  is  a  very  small  portion 
indeed. 

139.  How  Light  and  Heat  Travel  from  the  Sun  to  Us. 
Astronomers  tell  us  that  the  sun  —  the  chief  source  of  heat  and 
light  — -is  93,000,000  miles  away  from  us ;  that  is,  so  far  distant 
that  the  fastest  express  train  would  require  about  1 76  years  to 
reach  the  sun.  How  do  heat  and  light  travel  through  this 
vast  abyss  of  space  ? 

A  quiet  pool  and  a  pebble  will  help  to  make  it  clear  to  us. 
If  we  throw  a  pebble  into  a  quiet  pool  (Fig.  90),  waves  orrip- 


FIG.  90.  —  Waves  formed  by  a  pebble. 

pies  form  and  spread  out  in  all  directions,  gradually  dying  out 
as  they  become  more  and  more  distant  from  the  pebble.  It 
is  a  strange  fact  that  while  we  see  the  ripple  moving  farther 
and  farther  away,  the  particles  of  water  are  themselves  not 
moving  outward  and  away,  but  are  merely  bobbing  up  and 
down,  or  are  vibrating.  If  you  wish  to  be  sure  of  this,  throw 
the  pebble  near  a  spot  where  a  chip  lies  quiet  on  the  smooth 
pond.  After  the  waves  form,  the  chip  rides  up  and  down 
with  the  water,  but  does  not  move  outward ;  if  the  water  it- 


CL.    GEN.    SCI. —  10 


146  HEAT  AND  LIGHT  AS  COMPANIONS 

self  were  moving  outward,  it  would  carry  the  chip  with  it, 
but  the  water  has  no  forward  motion,  and  hence  the  chip  as- 
sumes the  only  motion  possessed  by  the  water,  that  is,  an 
up-and-down  motion.  Perhaps  a  more  simple  illustration  is 
the  appearance  of  a  wheat  field  or  a  lawn  on  a  windy  day  ;  the 
wind  sweeps  over  the  grass,  producing  in  the  grass  a  wave  like 
the  water  waves  of  the  ocean,  but  the  blades  of  grass  do  not 
move  from  their  accustomed  place  in  the  ground,  held  fast 
as  they  are  by  their  roots. 

If  a  pebble  is  thrown  into  a  quiet  pool,  it  creates  ripples  or 
waves  which  spread  outward  in  all  directions,  but  which  soon 
die  out,  leaving  the  pool  again  placid  and  undisturbed.  If 
now  we  could  quickly  withdraw  the  pebble  from  the  pool,  the 
water  would  again  be  disturbed  and  waves  would  form.  If 
the  pebble  were  attached  to  a  string  so  that  it  could  be  dropped 
into  the  water  and  withdrawn  at  regular  intervals,  the  waves 
would  never  have  a  chance  to  disappear,  because  there  would 
always  be  a  regularly  timed  definite  disturbance  of  the  water. 
Learned  men  tell  us  that  all  hot  bodies  and  all  luminous  bodies 
are  composed  of  tiny  particles,  called  molecules,  which  move 
unceasingly  back  and  forth  with  great  speed.  In  Section  95 
we  saw  that  the  molecules  of  all  substances  move  un- 
ceasingly ;  their  speed,  however,  is  not  so  great,  nor  are  their 
motions  so  regularly  timed  as  are  those  of  the  heat-giving  and 
the  light-giving  particles.  As  the  particles  of  the  hot  and 
luminous  bodies  vibrate  with  great  speed  and  force  they 
violently  disturb  the  medium  around  them,  and  produce  a 
series  of  waves  similar  to  those  produced  in  the  water  by 
the  pebble.  If,  however,  a  pebble  is  thrown  into  the  water 
very  gently,  the  disturbance  is  slight,  sometimes  too  slight  to 
throw  the  water  into  waves ;  in  the  same  way  objects  whose 
molecules  are  in  a  state  of  gentle  motion  do  not  produce 
light. 


HOW  HEAT  AND  LIGHT  DIFFER  147 

The  particles  of  heat-giving  and  light-giving  bodies  are  in 
a  state  of  rapid  vibration,  and  thereby  disturb  the  sur- 
rounding medium,  which  transmits  or  conveys  the  disturb- 
ance to  the  earth  or  to  other  objects  by  a  train  of  waves. 
When  these  waves  reach  their  destination,  the  sensation  of 
light  or  heat  is  produced. 

We  see  the  water  waves,  but  we  can  never  see  with  the  eye 
the  heat  and  light  waves  which  roll  in  to  us  from  that  far- 
distant  source,  the  sun.  We  can  be  sure  of  them  only  through 
their  effect  on  our  bodies,  and  by  the  visible  work  they  do. 

140.  How  Heat  and  Light  Differ.  If  heat  and  light  are 
alike  due  to  the  regular,  rapid  motion  of  the  particles  of  a 
body,  and  are  similarly  conveyed  by  waves,  how  is  it,  then, 
that  heat  and  light  are  apparently  so  different  ? 

Light  and  heat  differ  as  much  as  the  short,  choppy  waves 
of  the  ocean  and  the  slow,  long  swell  of  the  ocean,  but  not 
more  so.  The  sailor  handles  his  boat  in  one  way  in  a  choppy 
sea  and  in  a  different  way  in  a  rolling  sea,  ior  he  knows 
that  these  two  kinds  of  waves  act  dissimilarly.  The  long, 
slow  swell  of  the  ocean  would  correspond  with  the  longer, 
slower  waves  which  travel  out  from  the  sun,  and  which  on 
reaching  us  are  interpreted  as  heat.  The  shorter,  more  fre- 
quent waves  of  the  ocean  would  typify  the  short,  rapid  waves 
which  leave  the  sun,  and  which  on  reaching  us  are  interpreted 
as  light. 


CHAPTER   XV 

ARTIFICIAL  LIGHTING 

141.  We  seldom  consider  what  life  would  be  without  our 
wonderful  methods  of  illumination  which  turn  night  into  day, 
and  prolong  the  hours  of  work  and  pleasure.     Yet  it  was  not 
until  the  nineteenth  century  that  the  marvelous  change  was 
made  from  the  short-lived  candle  to  the  more  enduring  oil  lamp. 
Before  the  coming  of  the  lamp,  even  in  large  cities  like  Paris, 
the  only  artificial  light  to  guide  the  belated  traveler  at  night 
was  the  candle  required  to  be  kept  burning  in  an  occasional 
window. 

With  the  invention  of  the  kerosene  lamp  came  more  effi- 
cient lighting  of  home  and  street,  and  with  the  advent  of  gas 
and  electricity  came  a  light  so  effective  that  the  hours  of  busi- 
ness, manufacture,  and  pleasure  could  be  extended  far  beyond 
the  setting  of  the  sun. 

The  production  of  light  by  candle,  oil,  and  gas  will  be  con- 
sidered in  the  following  paragraphs,  while  illumination  by 
electricity  will  be  reserved  for  a  later  Chapter. 

142.  The  Candle.    Candles  were  originally  made  by  dipping 
a  wick  into  melting  tallow,  withdrawing  it,  allowing  the  ad- 
hered tallow  to  harden,  and  repeating  the  dipping  until  a  sat- 
isfactory thickness  was  obtained.     The  more  modern  method 
consists  in  pouring  a  fatty  preparation  into  a  mold,  at  the  cen- 
ter of  which  a  wick  has  been  placed. 

The  wick,  when  lighted,  burns  for  a  brief  interval  with  a 
faint,  uncertain  light ;  almost  immediately,  however,  the  inten- 

148 


OIL  I49 

sity  of  the  light  increases  and  the  illumination  remains  good 
as  long  as  the  candle  lasts.  The  heat  of  the  burning  wick 
melts  the  fatty  substance  near  it,  and  this  liquid  fat  is  quickly 
sucked  up  into  the  burning  wick.  The  heat  of  the  flaming 
wick  is  sufficient  to  change  this  liquid  into  a  gas ;  that  is,  to 
vaporize  the  liquid,  and  furthermore  to  set  fire  to  the  gas 
thus  formed. 

Small  particles  of  carbon  are  likewise  set  free,  and  these,  on 
coming  in  contact  with  the  oxygen  of  the  surrounding  air,  glow 
with  an  intense  heat  and  add  to  the  luminosity  of  the  candle 
flame.  In  order  that  the  gases  may  burn  and  the  solid  par- 
ticle glow,  a  plentiful  supply  of  oxygen  is  necessary.  If  the 
quantity  of  air  is  insufficient,  the  carbon  particles  remain  un- 
burned  and  form  soot.  A  lamp  "smokes"  when  the  air  which 
reaches  the  wick  is  insufficient  to  burn  to  incandescence  the 
rapidly  formed  carbon  particles ;  this  explains  the  danger  of 
turning  a  lamp  wick  toq^  high  and  producing  more  carbon 
particles  than  can  be  oxidized  by  the  air  admitted  through 
the  lamp  chimney. 

143.  Oil.  The  most  widely  used  illuminating  oils  are  kero- 
sene and  gasoline,  both  of  which  are  obtained  from  crude  pe- 
troleum, a  dark,  oily  liquid  occurring  in  the  earth.  •  The  chief 
oil-producing  regions  of  the  United  States  are  California, 
Oklahoma,  Illinois,  West  Virginia,  Ohio,  Texas,  Pennsylvania, 
and  Louisiana. 

The  crude  petroleum  as  it  is  taken  from  the  earth  is  a  mix- 
ture of  hydrocarbons ;  that  is,  compounds  of  hydrogen  and 
carbon  in  varying  combinations.  The  hydrocarbons  must  not 
be  confounded  by  the  pupil  with  carbohydrates,  Section  61, 
compounds  in  which  carbon  is  combined  wjth  hydrogen  and 
oxygen  in  definite  proportions. 

Crude  petroleum  is  separated  into  its  various  constituents 
by  distillation,  and  for  this  purpose  iron  retorts  connected  with 


150  ARTIFICIAL  LIGHTING 

condensers  and  tanks  to  receive  various  distillates  are  used. 
When  heat  is  applied  to  the  crude  liquid  in  the  retort,  the 
various  constituents  are  affected  differently;  the  substances 
in  the  crude  petroleum  which  have  a  low  boiling  point  vaporize 
first  and  distill  over  into  the  condensers.  As  the  temperature 
of  the  liquid  in  the  retort  rises,  substances  with  higher  boiling 
points  distill  over  and  condense ;  as  the  temperature  rises 
still  higher,  various  other  compounds  are  driven  off,  and  at  a 
very  high  temperature  all  that  is  left  in  the  retort  is  a  dark, 
thick  mass  called  coke. 

The  liquids  which  distill  over  at  different  temperatures 
are  unlike  in  character,  and  serve  widely  different  purposes. 
Among  the  products  obtained  in  this  way  by  the  fractional 
distillation  of  crude  petroleum  are  benzine,  gasoline,  kerosene 
or  coal  oil,  lubricating  oils,  petrolatum,  and  paraffin.  Many 
of  the  products  thus  obtained  require  further  treatment  before 
they  are  of  real  commercial  value ;  kerosene,  for  example,  is 
freed  as  far  as  possible  of  its  objectionable  odor. 

144.  Illuminating  Gas.  Much  of  the  gas  which  illuminates 
our  houses  is  made  from  the  distillation  of  bituminous  coal. 
Soft  coal  is  placed  in  clay-lined  retorts  which  are  connected 
by  pipes  to  a  series  of  tanks.  When  the  coal  is  heated  to 
1200°  C.  or  more,  certain  substances  in  it  volatilize  and  pass 
through  an  exit  tube  into  a  trough  which  contains  water,  and 
is  called  the  hydraulic  main.  The  water  in  the  hydraulic 
main  condenses  some  of  the  tarry  matter  which  distilled  over 
from  the  coal,  and  purifies  the  uncondensed  and  insoluble 
gases  which  bubble  through  it.  From  the  hydraulic  main, 
the  uncondensed  and  insoluble  gaseous  products  pass  onward 
through  a  series  of  coils  where  they  are  cooled,  and  where,  as 
a  result  of  cooling,  further  condensation  occurs.  The  gas, 
which  at  the  lower  temperature  still  remains  uncondensed, 
passes  through  a  series  of  vessels  which  possess  devices  for 


GAS  FOR   COOKING  151 

ridding  it  of  impurities ;  and  finally  as  illuminating  gas  it 
makes  its  way  into  a  huge  gas  holder  from  which  it  is  dis- 
tributed through  underground  service  pipes  to  the  buildings 
where  it  is  burned  for  light  or  heat. 

In  practice  the  gas  holder  is  constructed  with  a  sliding  top, 
which  rises  as  gas  enters  by  the  supply  pipes,  and  falls  when 
gas  leaves  through  the  service  pipes.  By  this  arrangement 
the  pressure  of  the  gas  within  the  tank  is  kept  constant,  and 
the  flow  through  the  service  pipes  remains  uniform  at  all  times. 
The  quantity  of  illuminating  gas  manufactured  in  the  United 
States  is  enormous,  amounting  to  more  than  68,000,000,000 
cubic  feet  per  year. 

145.  Gas  for  Cooking.  If  a  cold  object  is  held  in  the  bright 
flame  of  an  ordinary  gas  jet,  it  becomes  covered  with  soot,  or 
particles  of  unburned  carbon.  Although  the  flame  is  sur- 
rounded by  air,  the  central  portion  of  it  does  not  receive  suf- 
ficient oxygen  to  burn  up  the  numerous  carbon  particles  con- 
stantly thrown  off  by  the  burning  gas,  and  hence  many  carbon 
particles  remain  in  the  flame  as  glowing,  incandescent  masses. 
That  some  unburned  carbon  is  present  in  a  flame  is  shown  by 
the  fact  that  whenever  a  cold  object  is  held  in  the  flame, 
it  becomes  "  smoked  "  or  covered  with  soot.  If  enough  air 
were  supplied  to  the  flame  to  burn  up  the  carbon  as  fast  as 
it  was  set  free,  there  would  be  no  deposition  of  soot  on  ob- 
jects held  over  the  flame  or  in  it,  because  the  carbon  would 
be  transformed  into  gaseous  matter. 

Unburned  carbon  would  be  objectionable  in  cooking  stoves 
where  utensils  are  constantly  in  contact  with  the  flame,  and 
for  this  reason  cooking  stoves  are  provided  with  an  arrange- 
ment by  means  of  which  additional  air  is  supplied  to  the 
burning  gas  in  quantities  adequate  to  insure  complete  com- 
bustion of  the  rapidly  formed  carbon  particles.  An  opening 
is  made  in  the  tube  through  which  gas  passes  to  the  burner, 


152  ARTIFICIAL   LIGHTING 

and  as  the  gas  moves  past  this  opening,  it  carries  with  it  a 
draft  of  air.  These  openings  are  visible  on  all  gas  stoves, 
and  should  be  kept  clean  and  free  of  clogging,  in  order  to 
insure  complete  combustion.  So  long  as  the  supply  of  air 
is  sufficient,  the  flame  burns  with  a  dull  blue  color,  but  when 
the  supply  falls  below  that  needed  for  complete  burning  of 
the  carbon,  the  blue  color  disappears,  and  a  yellow  flame  takes 
its  place,  and  with  the  yellow  flame  the  deposition  of  soot  is 
inevitable. 

146.  By-products  of  Coal  Gas.     Many  important  products 
besides  illuminating  gas  are  obtained  from  the  distillation  of 
soft  coal.     Ammonia  is  made  from  the  liquids  which  collect  in 
the  condensers  ;  anilin,  the  source  of  exquisite  dyes,  is  made 
from  the  thick,  tarry  distillate,  and  coke  is  the  residue  left  in 
the  clay  retorts.     The  coal  tar  yields  not  only  anilin,  but  also 
carbolic  acid  and  naphthalene,  both  of  which  are  commercially 
valuable,  the  former  as  a  widely  used  disinfectant,  and  the 
latter  as  a  popular  moth  preventive. 

From  a  ton  of  good  gas-producing  coal  can  be  obtained 
about  10,000  cubic  feet  of  illuminating  gas,  and  as  by-prod- 
ucts 6  pounds  of  ammonia,  12  gallons  of  coal  tar,  and  1300 
pounds,  of  coke. 

147.  Natural  Gas.     Animal  and  vegetable  matter  buried 
in  the  depth  of  the  earth  sometimes  undergoes  natural  distil- 
lation, and  as  a  result,  gas  is  formed.     The  gas  produced  in 
this  way  is  called  natural  gas.     It  is  a  cheap  source  of  illu- 
mination, but  is  found  in  relatively  few  localities  and  only  in 
limited  quantity. 

148.  Acetylene.     In  1892  it  was  discovered  that  lime  and 
coal  fused  together  in  the  intense  heat  of  the  electric  furnace 
formed  a  crystalline,  metallic-looking  substance  called  calcium 
carbide.     As  a  result  of  that  discovery,  this  substance  was 
soon  made  on   a  large  scale  and  sold  at  a  moderate  price. 


ACETYLENE  153 

The  cheapness  of  calcium  carbide  has  made  it  possible  for  the 
isolated  farmhouse  to  discard  oil  lamps  and  to  have  a  private  gas 
system.  When  the  hard,  gray  crystals  of  calcium  carbide  are 
put  in  water,  they  give  off  acetylene,  a  colorless  gas  which  burns 
with  a  brilliant  white  flame.  If  bits  of  calcium  carbide  are 
dropped  into  a  test  tube  containing  water,  bubbles  of  gas  will  be 
seen  to  form  and  escape  into  the  air,  and  the  escaping  gas  may 
be  ignited  by  a  burning  match  held  near  the  mouth  of  the 
test  tube.  When  chemical  action  between  the  water  and  car- 
bide has  ceased,  and  gas  bubbles  have  stopped  forming,  slaked 
lime  is  all  that  is  left  of  the  dark  gray  crystals  which  were 
put  into  the  water. 

When  calcium  carbide  is  used  as  a  source  of  illumination, 
the  crystals  are  mechanically  dropped  into  a  tank  containing 
water,  and  the  gas  generated  is  automatically  collected  in  a 
small  sliding  tank,  whence  it  passes  through  pipes  to  the  va- 
rious rooms.  The  slaked  lime,  formed  while  the  gas  was  gen- 
erated, collects  at  the  bottom  of  the  tanks  and  is  removed 
from  time  to  time. 

The  cost  of  an  acetylene  generator  is  about  $50  for  a  small 
house,  and  the  cost  of  maintenance  is  not  more  than  that  of 
lamps.  The  generator  does  not  require  filling  oftener  than 
once  a  week,  and  the  labor  is  less  than  that  required  for  oil 
lamps.  In  a  house  in  which  there  were  twenty  burners,  the 
tanks  were  filled  with  water  and  carbide  but  once  a  fortnight. 
Acetylene  is  seldom  used  in  large  cities,  but  it  is  very  widely 
used  in  small  communities  and  is  particularly  convenient  in 
more  or  less  remote  summer  residences. 


CHAPTER   XVI 


MAN'S   WAY   OF   HELPING   HIMSELF 

149.    Labor-saving  Devices.     To    primitive  man   belonged 
more  especially  the  arduous  tasks  of  the  out-of-door  life :  the 

clearing  of  paths  through 
the  wilderness  ;  the  haul- 
ing of  material ;  the  break- 
ing up  of  the  hard  soil  of 
barren  fields  into  soft  loam 
ready  to  receive  the  seed ; 
the  harvesting  of  the  ripe 
grain,  etc. 

The     more     intelligent 

FIG.  91.  -  Prying  a  stone  out  of  the  ground.      r aces     among      men      SOOn 

learned  to  help  them- 
selves in  these  tasks.  For  example,  our  ancestors  in  the  field 
soon  learned  to  pry  stones  out  of  the  ground  (Fig.  91) 
rather  than  to  undertake  the  almost  impossible  task  of  lifting 
them  out  of  the  earth  in  which  they  were  embedded  ;  to  swing 
fallen  trees  away  from  a  path  by  means  of  rope  attached  to 
one  end  rather  than  to  attempt  to  remove  them  single-handed  ; 
to  pitch  hay  rather  than  to  lift  it ;  to  clear  a  field  with  a  rake 
rather  than  with  the  hands  ;  to  carry  heavy  loads  in  wheelbar- 
rows (Fig.  92)  rather  than  on  the  shoulders;  to  roll  barrels 
up  a  plank  (Fig.  93)  and  to  raise  weights  by  ropes.  In  every 
case,  whether  in  the  lifting  of  stones,  or  the  felling  of  trees, 
or  the  transportation  of  heavy  weights,  or  the  digging  of 
the  ground,  man  used  his  brain  in  the  invention  of  mechanical 

154 


WHEN  DO    WE   WORK? 


155 


devices  which  would    relieve    muscular    strain   and    lighten 

physical  labor. 

If  all  mankind  had  depended  upon  physical  strength  only, 

the  world  to-day  would  be 

in  the  condition  prevalent 

in    parts  of   Africa,  Asia, 

and    South     America, 

where   the  natives    loosen 

the  soil  with   their    hands 

or  with  crude   implements 

(Fig.    94),    and    transport 

huge     weights     on     their 

shoulders  and  heads. 

Any  mechanical  device 

(Figs.  95  and  96),  whereby 

man's    work  can  be  more 

conveniently  done,  is  called 

a  machine ;  the  machine  it- 
self never  does  any  work  — 

it  merely  enables  man  to  use  his  own  efforts  to  better  advantage. 
150.  When  do  we  Work  ?     Whenever,  as  a  result  of  effort  or 

force,  an  object  is 
moved,  work  is  done. 
If  you  lift  a  knapsack 
from  the  floor  to  the 
table,  you  do  work  be- 
cause you  use  force 
and  move  the  knap- 
sack through  a  distance 
equal  to  the  height  of 

the  table.     If  the  knapsack  were  twice  as  heavy,  you  would 

exert  twice  as  much  force  to  raise  it  to  the  same  height,  and 

hence  you  would    do   double  thje  work.     If  you  raised  the 


FIG.  92.  — The  wheelbarrow  lightens  labor. 


FIG.  93-  Rolling  barrels  up  a  plank. 


156  MAWS   WAY  OF  HELPING  HIMSELF 

knapsack  twice  the  distance,  —  say  to  your  shoulders  instead 
of  to  the  level  of  the  table,  —  you  would  do  twice  the  work, 

because  while  you  would 
exert  the  same  force  you 
would. continue  it  through 
double  the  distance. 

Lifting  heavy  weights 
through  great  distances  is 
not  the  only  way  in  which 
work  is  done.  Painting, 
chopping  wood,  hammer- 
ing, plowing,  washing, 
scrubbing,  sewing,  are  all 
forms  of  work.  In  painting, 
the  moving  brush  spreads 

FIG.  94.  —Crude  method  of  farming.  .,  r  • 

paint  over  a  surface;  in 

chopping  wood,  the  descending  ax  cleaves  the  wood  asunder ;  in 
scrubbing,  the  wet  mop  rubbed  over  the  floor  carries  dirt  away ; 
in  every  conceivable  form  of  work,  force  and  motion  occur. 

A  man  does  work  when  he  walks,  a  woman  does  work 
when  she  rocks  in  a  chair  — although  here  the  work  is  less 
than  in  walking.  On  a  windy  day  the  work  done  in  walking 
is  greater  than  normal.  The  wind  resists  our  progress,  and 
we  must  exert  more  force  in  order  to  cover  the  same  distance. 
Walking  through  a  plowed  or  rough  field  is  much  more  tiring 
than  to  walk  on  a  smooth  road,  because,  while  the  distance 
covered  may  be  the  same,  the  effort  put  forth  is  greater,  and 
hence  more  work  is  done.  Always  the  greater  the  resistance 
encountered,  the  greater  the  force  required,  and  hence  the 
greater  the  work  done. 

The  work  done  by  a  boy  who  raises  a  5-pound  knapsack  to 
his  shoulder  would  be  5x4,  or  20,  providing  his  shoulders 
were  4  feet  from  the  ground. 


MACHINES 


157 


The  amount  of  work  done  depends  upon  the  force  used 
and  the  distance  covered  (sometimes  called  displacement),  and 
hence  we  can  say  that 


or 


Work  =  force  multiplied  by  distance, 
J>F  =     x  d. 


151.  Machines.  A  glance  into 
our  machine  shops,  our  factories, 
and  even  our  homes  shows  how 
widespread  is  the  use  of  complex 
machinery.  But  all  machines,  how- 
ever complicated  in  appearance,  are 
in  reality  but  modifications  and  com- 
binations of  one  or  more  of  four 
simple  machines  devised  long  ago 
by  our  remote  ancestors.  These 
simple  devices  are  known  to-day,  as 
(i)  the  lever, represented  by  a  crow- 
bar, a  pitchfork ;  (2)  the  inclined 
plane,  represented  by  the  plank 
upon  which  barrels  are  rolled  into  a 
wagon  ;  (3)  the  pulley,  represented 
by  almost  any  contrivance  for  the 
raising  of  furniture  to  upper  stories  ; 
(4)  the  wheel  and  axle,  represented 
by  cogwheels  and  coffee  grinders. 

Suppose  a  6oo-pound  bowlder  which  is  embedded  in  the 
ground  is  needed  for  the  tower  of  a  building.  The  problem 
of  the  builder  is  to  get  the  heavy  bowlder  out  of  the  ground, 
to  load  it  on  a  wagon  for  transportation,  and  finally  to  raise 
it  to  the  tower.  Obviously,  he  cannot  do  this  alone  ;  the 
greatest  amount  of  force  of  which  he  is  capable  would  not 
suffice  to  accomplish  any  one  of  these  tasks.  How  then  does 


FlG.  95.  —  Primitive   method   of 
grinding  corn. 


158  MAWS  WAY  OF  HELPING  HIMSELF 

he  help  himself  and  perform  the  impossible  ?  Simply,  by 
the  use  of  some  of  the  machine  types  mentioned  above,  illus- 
trations of  which  are 
known  in  a  general  way  to 
every  schoolboy.  The  very 
knife  with  which  a  stick  is 
whittled  is  a  machine. 

152.  The  Lever.  Balance 
a  foot  rule,  containing  a 
hole  at  its  middle  point  F, 
as  shown  in  Figure  97.  If 
now  a  weight  of  I  pound 
is  suspended  from  the  bar 
at  some  point,  say  12,  the 
balance  is  disturbed,  and 

FIG.  96,-Separating  rice  grains  by  flailing.        the   bar     SwinSS    ab°Ut   the 

point  -F  as  a  center.     The 

balance  can  be  regained  by  suspending  an  equivalent  weight  at 
the  opposite  end  of  the  bar,  or  by  applying  a  2-pound  weight 
at  a  point  3  inches  to  the  left  of  F.  In  the  latter  case  a  force 
of  i  pound  actually  balances  a  force  of  2  pounds,  but  the 
i-pound  weight  is  twice  as  far  from  the  point  of  suspension  as 
is  the  2-pound  weight.  The  small  weight  makes  up  in  dis- 
tance what  it  lacks  in 
magnitude.  {-+-*-  3  4  5  F  7  8  9  10  u  12 

Such  an  arrange- 
ment of  a  rod  or  bar  is 
called  a  lever.  In  any 
form  of  lever  there  are  Azlbs- 

only  three  things  to  be  FlG  ^ _  The  prindple  of  the  ]ever 

considered :    the  point 

where  the  weight  rests,  the  point  where  the  force   acts,  and 
the  point  called  the  fulcrum  about  which  the  rod  rotates. 


APPLICATIONS   OF  THE  LEVER  159 

The  distance  from  the  force  to  the  fulcrum  is  called  the 
force  arm.  The  distance  from  the  weight  to  the  fulcrum  is 
called  the  weight  arm  ;  and  it  is  a  law  of  levers,  as  well  as  of 
all  other  machines,  that  the  force  multiplied  by  the  length  of 
the  force  arm  must  equal  the  weight  multiplied  by  the  length 
of  the  weight  arm. 

Force  x  force  arm  =  weight  X  weight  arm. 

A  force  of  I  pound  at  a  distance  of  6,  or  with  a  force  arm  6, 
will  balance  a  weight  of  2  pounds  with  a  weight  arm  3 ;  that  is, 

1x6=2x3. 

Similarly  a  force  of  10  pounds  may  be  made  to  sustain  a 
weight  of  100  pounds,  providing  the  force  arm  is  10  times 
longer  than  the  weight  arm ;  and  a  force  arm  of  800  pounds, 
at  a  distance  of  10  feet  from  the  fulcrum,  may  be  made  to 
sustain  a  weight  of  8000  pounds,  providing  the  weight  is  I  foot 
from  the  fulcrum. 

153  Applications  of  the  Lever.  By  means  of  a  lever,  a  600- 
pound  bowlder  can  be  easily  pried  out  of  the  ground.  Let 
the  lever,  any  strong  metal  bar,  be  supported  on  a  stone  which 
serves  as  fulcrum  ;  then  if  a  man  exerts  his  force  at  the  end  of 
the  rod  somewhat  as  in  Figure  91  (p.  1 54),  the  force  arm  will  be 
the  distance  from  the  stone  or  fulcrum  to  the  end  of  the  bar, 
and  the  weight  arm  will  be  the  distance  from  the  fulcrum  to  the 
bowlder  itself.  The  man  pushes  down  with  a  force  of  100 
pounds,  but  with  that  amount  succeeds  in  prying  up  the  600- 
pound  bowlder.  If,  however,  you  look  carefully,  you  will  see 
that  the  force  arm  is  6  times  as  long  as  the  weight  arm,  so 
that  the  smaller  force  is  compensated  for  by  the  greater  dis- 
tance through  which  it  acts. 

At  first  sight  it  seems  as  though  the  man's  work  were  done 
for  him  by  the  machine.  But  this  is  not  so.  The  man  must 


160  MAN^S    WAY  OF  HELPING  HIMSELF 

lower  his  end  of  the  lever  3  feet  in  order  to  raise  the  bowlder 
6  inches  out  of  the  ground.  He  does  not  at  any  time  exert  a 
large  force,  but  he  accomplishes  his  purpose  by  exerting  a 
small  force  continuously  through  a  correspondingly  greater 
distance.  He  finds  it  easier  to  exert  a  force  of  100  pounds  con- 
tinuously until  his  end  has  moved  3  feet  rather  than  to  exert 
a  force  of  600  pounds  on  the  bowlder  and  move  it  6  inches. 

By  the  time  the  stone  has  been  raised  the  man  has  done  as 
much  work  as  though  the  stone  had  been  raised  directly,  but 
his  inability  to  put  forth  sufficient  muscular  force  to  raise 
the  bowlder  directly  would  have  rendered  impossible  a  result 
which  was  easily  accomplished  when  through  the  medium 
of  the  lever  he  could  extend  his  small  force  through  greater 
distance. 

154.  The  Wheelbarrow  as  a  Lever.  The  principle  of  the 
lever  is  always  the  same ;  but  the  relative  position  of  the  im- 
portant points  may  vary. 
For  example,  the  fulcrum 
is  sometimes  at  one  end, 
the  force  at  the  opposite 
end,  and  the  weight  to  be 
lifted  between  them. 

Suspend  a  stick  with  a 
hole  at  its  center  as  in 
Figure  98,  and  hang  a 


1 1  Ibs. 


FIG.  98.  -  A  slightly  different  form  of  lever.          4'POUnd  Weight  at    a    dlS- 

tance  of    I  foot  from  the 

fulcrum,  supporting  the  load  by  means  of  a  spring  balance  2 
feet  from  the  fulcrum.  The  pointer  on  the  spring  balance 
shows  that  the  force  required  to  balance  the  4-pound  load  is 
but  2  pounds. 

The  force  is  2  feet  from  the  fulcrum,  and  the  weight  (4) 
is  I  foot  from  the  fulcrum,  so  that 


THE   WHEELBARROW  AS  A   LEVER 


or 


Force  x  distance  =  Weight  x  distance, 
2x2  =  4x1. 


FIG.  99.  —  The  wheelbarrow  lightens  labor. 


Move  the  4-pound  weight  so  that  it  is  very  near  the  ful- 
crum, say  but  6  inches  from 

it ;  then  the  spring  balance 

registers    a   force   only  one 

fourth  as  great  as  the  weight 

which  it  suspends.     In  other 

words  a  force  of  I  at  a  dis- 
tance of  24  inches  (2  feet)  is 

equivalent  to  a  force  of  4  at 

a  distance  of  6  inches. 
One   of  the   most  useful 

levers  of  this    type    is    the 

wheelbarrow  (Fig.  99).    The 

fulcrum  is  at  the  wheel,  the 

force  is  at  the  handles,  the   weight  is  on  the  wheelbarrow. 

If  the  load  is  halfway  from  the  fulcrum  to  the  man's  hands, 

the  man  will  have  to  lift 
with  a  force  equal  to 
one  half  the  load.  If 
the  load  is  one  fourth  as 
far  from  the  fulcrum  as 
the  man's  hands,  he  will 
need  to  lift  with  a  force 
only  one  fourth  as  great 
as  that  of  the  load. 

This  shows  that  in 
loading  a  wheelbarrow, 
it  is  important  to  ar- 
range the  load  as  near 

FIG.  ioo.  —  A  modified  wheelbarrow.  to  the  wheel  as  possible. 

CL.    GEN.    SCI. —  II 


162 


MAWS   WAY  OF  HELPING  HIMSELF 


FIG.  101.  —  The  nutcracker  is  a  lever. 


The  nutcracker  (Fig.  .101)  is  an  illustration  of  a  double 
lever  of  the  wheelbarrow  kind ;  the  nearer  the  nut  is  to  the 

fulcrum,      the     easier     the 
cracking. 

Hammers  (Fig.  102),  tack- 
lifters,  scissors,  forceps,  are 
important  levers,  and  if  you 
will  notice  how  many  differ- 
ent levers  (fig.  103)  are  used 
by  all  classes  of  men,  you  will 
understand  how  valuable  a  machine  this  simple  device  is. 

155.  The  Inclined  Plane.  A  man  wishes  to  load  the  600- 
pound  bowlder  on  a  wagon,  and  proceeds  to  do  it  by  means 
of  a  plank,  as  in  Figure  93.  Such  an  arrangement  is  called 
an  inclined  plane. 

The  advantage  of  an  inclined  plane  can  be  seen  by  the 
following  experiment.  Select  a  smooth  board  4  feet  long 
and  prop  it  so  that  the  end  A  (Fig.  104)  is  I  foot  above  the 
level  of  the  table ;  the  length  of  the  incline  is  then  4  times 
as  great  as  its  height.  Fasten  . 
a  metal  roller  to  a  spring  bal- 
ance and  observe  its  weight. 
Then  pull  the  roller  uniformly 
upward  along  the  plank  and 
notice  what  the  pull  is  on  the 
balance,  being  careful  always  to 
hold  the  balance  parallel  to  the 
incline. 

When    the     roller    is    raised  FlG  '  ioa  _  The  hand  exerts  a  small 

along    the     incline,    the     balance       force  over  a  long  distance  and  draws 
.   ,  ni  r         .1.1        out  a  nail. 

registers  a  pull  only  one  fourth 

as  great  as  the  actual  weight  of  the  roller.     That  is,  when  the 

roller  weighs  12,  a  force  of  3  suffices  to  raise  it  to  the  height 


APPLICATION 


163 


A  along  the  incline ;  but  the  smaller  force  must  be  applied 
throughout  the  entire  length  of  the  incline.  In  many  cases, 
it  is  -  preferable  to  exert  a 
force  of  30  pounds,  for  ex- 
ample, over  the  distance 
CA  than  a  force  of  120 
pounds  over  the  shorter 
distance  BA. 

Prop  the  board  so  that 
the  end  A  is  2  feet  above 
the  table  level ;  that  is,  ar- 
range the  inclined  plane  in 
such  a  way  that  its  length 
is  twice  as  great  as  its 

height.       In    that    Case    the   FlG.  103.  —  Primitive  man  tried  to  lighten  his 

steady  pull  on  the  balance 

will  be  one  half  the  weight  of  the  roller;  or  a  force  of  6 

pounds  will  suffice  to  raise  the  12-pound  roller. 

The  steeper  the  incline,  the  more  force  necessary  to  raise  a 

weight;  whereas  if  the  incline  is  small,  the  necessary  lifting 

force  is  greatly  reduced. 
On  an  inclined  plane  whose 
length  is  ten  times  its 
height,  the  lifting  force  is 
reduced  to  one  tenth  the 
weight  of  the  load.  The 
advantage  of  an  incline  de- 
pends upon  the  relative 

FIG.  104.  —  Less  force  is  required  to  raise  the    length      and      height,      Or    IS 
roller  along  the  incline  than  to  raise  it  to  A   equal    to     the     ratio     Qf     the 

directly. 

length  to  the  height. 

156.    Application.     By  the  use  of  an  inclined  plank  a  strong 
man  can  load  the  6oo-pound  bowlder  on  a  wagon.     Suppose 


1 64  MAN^S   WAY  OF  HELPING  HIMSELF 

the  floor  of  the  wagon  is  2  feet  above  the  ground,  then  if  a 
6-foot  plank  is  used,  200  pounds  of  force  will  suffice  to  raise 
the  bowlder ;  but  the  man  will  have  to  push  with  this  force 
against  the  bowlder  while  it  moves  over  the  entire  length 
of  the  plank. 

Since  work  is  equal  to  force  multiplied  by  distance,  the 
man  has  done  work  represented  by  200  x  6,  or  1200.  This 
is  exactly  the  amount  of  work  which  would  have  been  neces- 
sary to  raise  the  bowlder  directly.  A  man  of  even  enormous 
strength  could  not  lift  such  a  weight  (600  Ib.)  even  an  inch 
directly,  but  a  strong  man  can  furnish  the  smaller  force  (200) 
over  a  distance  of  6  feet;  hence,  while  the  machine  does 
not  lessen  the  total  amount  of  work  required  of  a  man,  it 
creates  a  new  distribution  of  work  and  makes  possible,  and 
even  easy,  results  which  otherwise  would  be  impossible  by 
human  agency. 

157.  Railroads  and  Highways.  The  problem  of  the  incline 
is  an  important  one  to  engineers  who  have  under  their  direc- 
tion the  construction  of  our  highways  and  the  laying  of  our 
railroad  tracks.  It  requires  tremendous  force  to  pull  a  load 


FIG.  105.  —  A  well-graded  railroad  bed. 


RAILROADS  AND  HIGHWAYS  165 

up  grade,  and  most  of  us  are  familiar  with  the  struggling 
horse  and  the  puffing  locomotive.  For  this  reason  engineers, 
wherever  possible,  level  down  the  steep  places,  and  reduce 
the  strain  as  far  as  possible. 

The  slope  of  the  road  is  called  its  grade,  and  the  grade 
itself  is  simply  the  number  of  feet  the  hill  rises  per  mile.     A 


FlG.  106.  —  A  long,  gradual  ascent  is  better  than  a  shorter,  steeper  one. 

road  a  mile  long  (5280  feet)  has  a  grade  of  132  if  the  crest  of 
the  hill  is  132  feet  above  the  level  at  which  the  road  started. 

In  such  an  incline,  the  ratio  of  length  to  height  is  5280-1-  132, 
or  40;  and  hence  in  order  to  pull  a  train  of  cars  to  the  summit, 
the  engine  would  need  to  exert  a  continuous  pull  equal  to 
one  fortieth  of  the  combined  resistance  of  the  train. 

If,  on  the  other  hand,  the  ascent  had  been  gradual,  so  that 
the  grade  was  66  feet  per  mile,  a  pull  from  the  engine  of  one 
eightieth  of  the  combined  resistance  would  have  sufficed  to 
land  the  train  of  cars  at  the  crest  of  the  grade. 


1 66  MAN'S   WAY  OF  HELPING  HIMSELF 

Because  of  these  facts,  engineers  spend  great  sums  in 
grading  down  railroad  beds,  making  them  as  nearly  level  as 
possible.  In  mountainous  regions,  the  topography  of  the 
land  prevents  the  elimination  of  all  steep  grades,  but  never- 
theless the  attempt  is  always  made  to  follow  the  easiest  grades. 

158.  The  Wedge.     If  an  inclined 
plane    is     pushed    underneath     or 
within    an    object,   it  serves    as    a 
wedge.     Usually  a  wedge  consists 
of  two  inclined  planes  (Fig.  107). 

A  chisel  and  an  ax  are  illustra- 
tions of  wedges.  Perhaps  the  most 
universal  form  of  a  wedge  is  our 
common  pin.  Can  you  explain  how 

FlG.  107.  —  Bv  means  of  a  wedge,          . 

the  stump  is  split.  this  IS  a  Wedge  ? 

159.  The  Screw.    Another  valua- 
ble and  indispensable  form  of  the  inclined  plane  is  the  screw. 
This  consists  of  a  metal  rod  around  which  passes  a  ridge,  and 
Figure  108  shows  clearly  that  a  screw  is  simply  a 

rod  around  which  (in  effect)  an  inclined  plane  has 
been  wrapped. 

The  ridge  encircling  the  screw  is  called  the 
thread,  and  the  distance  between  two  successive 
threads  is  called  the  pitch.  It  is  easy  to  see  that 
the  closer  the  threads  and  the  smaller  the  pitch, 

FIG.     108.— 

the  greater  the  advantage  of  the  screw,  and  hence      A  screw  as 
the  less  force   needed  in   overcoming   resistance.      a     SImPIe 

0  machine. 

A  corkscrew  is  a  familiar  illustration  of  the  screw. 

160.  Pulleys.  The  pulley,  another  of  the  machines,  is 
merely  a  grooved  wheel  around  which  a  cord  passes.  It  is 
sometimes  more  convenient  to  move  a  load  in  one  direction 
rather  than  in  another,  and  the  pulley  in  its  simplest  form 
enables  us  to  do  this.  In  order  to  raise  a  flag  to  the  top  of 


MOVABLE  PULLEYS 


I67 


a  mast,  it  is  not  necessary  to  climb  the  mast,  and  so  pull 

up  the  flag;  the  same  result  is  accomplished  much  more  easily 

by  attaching  the  flag  to  a  movable  string, 

somewhat  as  in    Figure   109,  and  pulling 

from  below.     As  the  string  is  pulled  down, 

the  flag  rises  and  ultimately  reaches  the 

desired  position. 

If  we  employ  a  stationary  pulley,  as  in 
Figure  109,  we  do  not  change  the  force,  be- 
cause the  force  required  to  balance  the  load 
is  as  large  as  the  load  itself.  The  only  ad- 
vantage is  that  a  force  in  one  direction  may 
be  used  to  produce  motion  in  the  opposite 
direction.  Such  a  pulley  is  known  as  a 
fixed  pulley. 

161.  Movable  Pulleys.  By  the  use  of 
a  movable  pulley,  we  are  able  to  support 
a  weight  by  a  force  equal  to  only  one  half 
the  load.  In  Figure  109,  the  downward 
pull  of  the  weight  and  the  downward  pull 
of  the  hand  are  equal ;  in  Figure  1 10,  the  FIG.  109.  — By  means  of 
spring  balance  supports  only  one  half  the  S^^p^dS«m^ 
entire  load,  the  remaining  half  being  borne  tion .in  the  opposite  di- 

i         i      T         i  1-11  ••  11         rection. 

by  the  hook  to  which  the  string  is  attached. 
The  weight  is  divided  equally  between  the  two  parts  of  the 
string  which  passes  around  the  pulley,  so  that  each  strand 
bears  only  one  half  of  the  burden. 

We  have  seen  in  our  study  of  the  lever  and  the  inclined 
plane  that  an  increase  in  force  is  always  accompanied  by  a  i 
decrease  in  distance,  and  in  the  case  of  the  pulley  we  naturally 
look  for  a  similar  result.  If  you  raise  the  balance  (Fig.  1 10)  12 
feet,  you  will  find  that  the  weight  rises  only  6  feet ;  if  you 
raise  the  balance  24  inches,  you  will  find  that  the  weight  rises 


168 


MAN^S   WAY  OF  HELPING  HIMSELF 


12  inches.     You  must  exercise  a  force  of  100  pounds  over  12 

feet  of  space  in  order  to  raise  a  weight  of  200  pounds  a  distance 
of  6  feet.  When  we  raise  100  pounds 
through  12  feet  or  200  pounds  through  6 
feet  the  total  work  done  is  the  same ;  but 
the  pulley  enables  those  who  cannot  furnish 
a  force  of  200  pounds  for  the  space  of  6 
feet  to  accomplish  the  task  by  furnishing 
100  pounds  for  the  space  of  12  feet. 

162.  Combination  of  Pulleys.  A  combi- 
nation of  pulleys  called  block  and  tackle  is 
used  where  very  heavy  loads  are  to  be 
moved.  In  Figure  1 1 1 
the  upper  block  of  pul- 
leys is  fixed,  the  lower 
block  is  movable,  and 
one  continuous  rope 
passes  around  the  vari- 
ous pulleys.  The  load  is 
supported  by  6  strands, 
and  each  strand  bears 
one  sixth  of  the  load. 

If  the  hand  pulls  with  a  force  of  I  pound 

at  P,  it  can  raise  a  load  of  6  pounds  at  W, 

but  the  hand  will  have  to  pull  downward 

6  feet  at  P  in  order  to  raise  the  load  at 

W  i  foot.     If  8  pulleys  were  used,  a  force 

equivalent  to  one  eighth  of  the  load  would 

suffice  to  move  W,  but  this  force  would  have 

to  be  exerted  over  a  distance  8  times  as 

great  as  that  through  which  W  was  raised. 

163.    Practical  Application.     In  our  childhood  many  of  us 

saw  with  wonder  the  appearance  and  disappearance  of  flags 


FIG.  no.  —  A  movable 
pulley  lightens  labor. 


FIG.  in.  —  An  effec- 
tive arrangement  of 
pulleys  known  as 
block  and  tackle. 


WHEEL  AND  AXLE 


169 


flying  at  the  tops  of  high  masts,  but  observation  soon  taught 
us  that  the  flags  were  raised  by  pulleys.  In  tenements,  where 
there  is  no  yard  for  the  family  washing,  clothes  often  appear 
flapping  in  mid-air.  This  seems  most  marvelous  until  we  learn 
that  the  lines  are  pulled  back  and  forth  by  pulleys  at  the  win- 
dow and  at  a  distant  support.  By  means  of  pulleys,  awnings  are 
raised  and  lowered,  and  the  use  of  pulleys  by  furniture  movers, 
etc.,  is  familiar  to  every  wide-awake  observer  on  the  streets. 

164.  Wheel  and  Axle.  The  wheel  and  axle  consists  of  a  large 
wheel  and  a  small  axle  so  fastened  that  they  rotate  together. 

When  the  large  wheel  makes 
one  revolution,  P  falls  a  dis- 
tance equal  to  the  circumfer- 
ence of  the  wheel.  While  P 
moves  downward,  W  likewise 
moves,  but  its  motion  is  upward, 
and  the  distance  it  moves  is 
small,  being  equal  only  to  the 
circumference  of  the  small  axle. 
But  a  small  force  at  P  will 
sustain  a  larger  force  at  W',  if 
the  circumference  of  the  large 

wheel  is  40  inches,  and  that  of  the  small  wheel  10  inches,  a 
load  of  100  at  Wean  be  sustained  by  a  force  of  25  at  P.  The  in- 
crease in  force  of  the  wheel  and  axle  depends  upon  the  relative 
size  of  the  two  parts,  that  is,  upon  the  circumference  of  wheel 
as  compared  with  circumference  of  axle,  and  since  the  ratio 
between  circumference  and  radius  is  constant,  the  ratio  of 
the  wheel  and  axle  combination  is  the  ratio  of  the  long 
radius  to  the  short  radius. 

For  example,  in  a  wheel  and  axle  of  radii  20  and  4, 
respectively,  a  given  weight  at  P  would  balance  5  times  as 
great  a  load  at  W. 


FIG.  112.  —  The  wheel  and  axle. 


MAN">S   WAY  OF  HELPING  HIMSELF 


165.  Application.  Windlass,  Cogwheels.  In  the  old-fash- 
ioned windlass  used  in  farming  districts,  the  large  wheel  is 
replaced  by  a  handle  which,  when  turned,  describes  a  circle. 
Such  an  arrangement  is  equivalent  to  wheel  and  axle  (Fig. 
112);  the  capstan  used  on  shipboard  for  raising  the  anchor 
has  the  same  principle.  The  kitchen  coffee  grinder  and  the 
meat  chopper  are  other  familiar  illustrations. 

Cogwheels  are  modifications  of  the  wheel  and  axle.  Teeth 
cut  in  A  fit  into  similar  teeth  cut  in  B,  and  hence  rotation  of 

A  causes  rotation  of  B.  Several 
revolutions  of  the  smaller  wheel, 
however,  are  necessary  in  order  to 
turn  the  larger  wheel  through  one 
complete  revolution ;  if  the  radius 
of  A  is  one  half  that  of  B,  two  revo- 
lutions of  A  will  correspond  to  one 
of  B\  if  the  radius  of  A  is  one  third 
that  of  B,  three  revolutions  of  A 
will  correspond  to  one  of  B. 

Experiment  demonstrates  that  a 
weight  W  attached  to  a  cogwheel  of 
radius  3  can  be  raised  by  a  force  P, 

equal  to  one  third  of  PFapplied  to  a  cogwheel  of  radius  I.  There 
is  thus  a  gr^eat  increase  in  force.  But  the  speed  with  which  Wis 
raised  is  only  one  third  the  speed  with  which  the  small  wheel  ro- 
tates, or  increase  in  power  has  been  at  the  decrease  of  speed. 
This  is  a  very  common  method  for  raising  heavy  weights 
by  small  force. 

Cogwheels  can  be  made  to  give  speed  at  the  decrease  of  force. 
A  heavy  weight  W  attached  to  B  will  in  its  slow  fall  cause 
rapid  rotation  of  A,  and  hence  rapid  rise  of  P.  It  is  true  that 
P,  the  load'f aised,  will  be  less  than  W,  the  force  exerted,  but  if 
speed  is  our  aim,  this  machine  serves  our  purpose  admirably. 


FIG.  113.  — Cogwheels. 


MEASUREMENT  OF   WORK 


171 


FlG.   114. —  By  means  of  a  belt,  motion 
can  be  transferred  from  place  to  place. 


An  extremely  important  form  of  wheel  and  'axle  is  that  in 
which  the  two  wheels  are  connected  by  belts  as  in  Figure  1 14. 
Rotation  of  W7  induces  rotation 
of  w,  and  a  small  force  at  W  is 
able  to  overcome  a  large  force 
at  w.  An  advantage  of  the 
belt  connection  is  that  power 
at  one  place  can  be  transmitted 
over  a  considerable  distance 
and  utilized  in  another  place. 

166.  Compound  Machines.  Out  of  the  few  simple  machines 
mentioned  in  the  preceding  Sections  has  developed  the  complex 
machinery  of  to-day.  By  a  combination  of  screw  and  lever,  for 
example,  we  obtain  the  advantage  due  to  each  device,  and  some 
compound  machines  have  been  made  which  combine  all  the  va- 
rious kinds  of  simple  machines,  and  in  this  way  multiply  their  me- 
chanical advantage  many  fold. 
A  relatively  simple  com- 
plex machine  called  the  crane 
(Fig.  116)  maybe  seen  almost 
any  day  on  the  street,  or 
wherever  heavy  weights  are 
being  lifted.  It  is  clear  that  a 
force  applied  to  turn  wheel  I 
causes  a  slower  rotation  of 
wheel  3,  and  a  still  slower 
rotation  of  wheel  4,  but  as  4 
rotates  it  winds  up  a  chain  and 


FIG. 


115.  —  A  simple    derrick 
weights. 


raising 


167.   Measurement  of  Work. 


slowly  raises  Q.     A  very  com- 
plex machine  is  that  seen  in 
Figure  117. 
In  Section   150,  -we  learned 


that    the    amount    of    work    done    depends    upon    the    force 


1/2  MAN'S   WAY  OF  HELPING   HIMSELF 

exerted,  and  the  distance  covered,  or  that  W  —  force  x  dis- 
tance.    A  man  who  raises  5  pounds  a  height  of  5  feet  does 

far  more  work  than  a 
man  who  raises  5  ounces 
a  height  of  5  inches, 
but  the  product  of  force 
by  distance  is  25  in  each 
case.  There  is  difficulty 
because  we  have  not 
selected  an  arbitrary 
unit  of  work.  The  unit 
of  work  chosen  and  in 
use  in  practical  affairs 
is  the  foot  pound,  and 
is  defined  as  the  work 

FIG.  116.- A  traveling  crane.  done    when     a     force     of 

I    pound    acts   through 

a  distance  of  I  foot.     A  man  who  moves  8  pounds  through 
6  feet  does  48  foot  pounds  of  work,  while  a  man  who  moves 


FIG    117. —  A  farm  engine  putting  in  a  crop. 

8  ounces  (|  pound)  through  6  inches  (\  foot)  does  only  one 
fourth  of  a  foot  pound  of  work. 


WASTE   U'ORK  AND   EFFICIENT   IVORK  173 

168.  The  Power  or  the  Speed  with  which  Work  is  Done.    A 

man  can  load  a  wagon  more  quickly  than  a  growing  boy. 
The  work  done*  by  the  one  is  equal  to  the  work  done  by  the 
other,  but  the  man  is  more  efficient,  because  the  time  required 
for  a  given  task  is  very  important. 

An  engine  which  hoists  a  5O-pound  weight  in  i  second  is 
much  more  efficient  than  a  man  who  requires  50  seconds  for 
the  same  task ;  hence  in  estimating  the  value  of  a  working 
agent,  whether  animal  or  mechanical,  we  must  consider  not 
only  the  work  done,  but  the  speed  with  which  it  is  done. 

The  rate  at  which  a  machine  is  able  to  accomplish  a  unit 
of  work  is  called  power,  and  the  unit  of  power  customarily 
used  is  the  horse  power. 

Any  power  which  can  do  550  foot  pounds  of  worjc  per 
second  is  said  to  be  one  horse  power  (H.P.).  This  unit  was 
chosen  by  James  Watt,  the  inventor  of  a  steam  engine.  Al- 
though called  a  horse  power  it  is  greater  than  the  power  of 
an  average  horse. 

An  ordinary  man  can  do  one  sixth  of  a  horse  power.  The 
average  locomotive  of  a  railroad  has  more  than  500  H. P.,  while 
the  engine  of  an  ocean  liner  may  have  as  high  as  70,000  H.P. 

169.  Waste  Work  and  Efficient  Work.     In  our  study  of  ma- 
chines we  omitted  a  factor  which  in  practical  cases  cannot  be 
ignored,  namely,  friction.     No  surface  can  be  made  perfectly 
smooth,  and  when  a  barrel  rolls  over  an  incline,  or  a  rope 
passes  over  a  pulley,  or  a  cogwheel  turns  its  neighbor,  there 
is  rubbing  and  slipping  and  sliding.     Motion  is  thus  hindered, 
and  the  effective  value  of  the  acting  force  is  lessened.     In  or- 
der to  secure   the  desired  result  it  is  necessary  to  apply  a 
force  in  excess  of  that  calculated.     This  extra  force,  which 
must  be  supplied  if  friction  is  to  be  counteracted,  is  in  reality 
waste  work ;  the  greater  the  friction,  the  greater  the  waste 
work,  and  the  less  we  actually  get  out  of  the  machine. 


174 


MAWS    WAY  OF  HELPING   HIMSELF 


If  the  force  required  by  a  machine  is  150  pounds,  while 
that  calculated  as  necessary  is  100  pounds,  the  loss  due  to 
friction  is  50  pounds,  and  the  machine,  instead  of  being 
thoroughly  efficient,  is  only  two  thirds  efficient. 

Machinists  make  every  effort  to  eliminate  from  a  ma- 
chine the  waste  due  to  friction,  leveling  and  grinding  to  the 
most  perfect  smoothness  and  adjustment  every  part  of  the 
machine.  When  the  machine  is  in  use,  friction  may  be  further 
reduced  by  the  use  of  lubricating  oil.  Friction  can  never 
be  totally  eliminated,  however,  and  machines  of  even  the  finest 
construction  lose  by  friction  one  tenth  of  their  efficiency,  while 
poorly  constructed  ones  lose  by  friction  as  much  as  one  half 
of  their  efficiency. 

170*  Man's  Strength  not  Sufficient  for  Machines.  A  machine, 
an  inert  mass  of  metal  and  wood,  cannot  of  itself  do  any  work, 

but  can  only  distribute  the 
energy  which  is  brought  to  it. 
Fortunately  it  is  not  neces- 
sary that  this  energy  should 
be  contributed  by  man  alone, 
because  the  store  of  energy 
possessed  by  him  is  very 
small  in  comparison  with  the 
energy  required  to  run  loco- 
motives, automobiles,  saw- 
mills, etc.  Perhaps  the  great- 


FIG. us.     Man's  strength 
for  heavy  work. 


est  value  of  machines  lies  in 
nsufficient  the  fact  that  they  enable  man 


to  perform  work  by  the  use  of 
energy  other  than  his  own. 

Figure  1  18  shows  one  way  in  which  aJiorse's  energy  can  be 
utilized  in  lifting  heavy  loads.  Even  the  fleeting  wind  has 
been  harnessed  by  man,  and,  as  in  the  windmill,  made  to  work 


MAWS  STRENGTH  NOT  SUFFICIENT  FOR  MACHINES   175 


for  him  (Fig.  119).  One  sees  dotted  over  the  country  wind- 
mills large  and  small,  and  in  Holland,  the  country  of  wind- 
mills, the  land- 
owner who  does 
not  possess  a 
windmill  is  poor 
indeed. 

For  genera- 
tions running 
water  from 
rivers,  streams, 
and  falls  has 
served  man  by 
carrying  his  logs 
downstream,  by 
turning  the 
wheels  of  his 
mill,  etc.*,  and  in 
our  own  day 
running  water  is 
used  as  an  in- 
direct source  of 
electric  lights 
for  street  and 

house      the      en-   FIG.  119.  —  The  windmill  pumps  water  into  the  troughs  where 

cattle  drink. 

ergy  of  the  fall- 
ing water  serving  to  rotate  the  armature  of  a  dynamo  (Sec- 
tion 310). 

A  more  constant  source  of  energy  is  that  available  from  the 
burning  of  fuel,  such  as  coal  and  oil.  The  former  is  the 
source  of  energy  in  locomotives,  the  latter  in  most  automobiles. 

In  the  following  Chapter  will  be  given  an  account  of  water, 
wind,  and  fuel  as  machine  feeders.  , 


CHAPTER    XVII 

THE   POWER   BEHIND   THE   ENGINE 

171.  Small  boys  soon  learn  the  power  of  running  water; 
swimming  or  rowing  downstream  is  easy,  while  swimming  or 
rowing  against  the  current  is  difficult,  and  the  swifter  the 
water,  the  easier  the  one  and  the  more  difficult  the  other; 
the  river  assists  or  opposes  us  as  we  go  with  it  or  against  it. 
The  water  of  a  quiet  pool  or  of  a  gentle  stream  cannot  do 
work,  but  water  which  is  plunging  over  a  precipice  or  dam,  or  is 
flowing  down  steep  slopes,  may  be  made  to  saw  wood,  grind 
our  corn,  light  our  streets,  run  our  electric  cars,  etc.  A 
waterfall,  or  a  rapid  stream,  is  a  great  asset  to  any  community, 
and  for  this  reason  should  be  carefully  guarded.  Water.power 
is  as  great  a  source  of  wealth  as  a  coal  bed  or  a  gold  mine. 

The  most  tremendous  waterfall  in  our  country  is  Niagara 
Falls,  which  every  minute  hurls  millions  of  gallons  of  water 
down  a  1 63-foot  incline.  The  energy  possessed  by  such  an 
enormous  quantity  of  water  flowing  at  such  a  tremendous 
speed  is  almost  beyond  everyday  comprehension,  and  would 
suffice  to  run  the  engines  of  many  cities  far  and  near.  Numer- 
ous attempts  to  buy  from  the  United  States  the  right  to  utilize 
some  of  this  apparently  wasted  energy  have  been  made  by 
various  commercial  companies.  It  is  fortunate  that  these 
negotiations  have  been  largely  fruitless,  because  much  devia- 
tion of  the  water  for  commercial  uses  and.  the  installation  of 
machinery  in  the  vicinity  of  the  famous  falls  would  greatly 

176 


WATER    W 'HEELS 


177 


detract  from  the  beauty  of  this  world-known  scene,  and  would 
rob  our  country  of  a  natural  beauty  unequaled  elsewhere. 
172.   Water  Wheels. 

120 
small 


In 


Figure 


FIG.  120.  —  A  mountain  stream  turns  the  wheels  of 
the  mill. 


the 

water  of   a   small  but 

rapid  mountain  stream 

is    made    to    rotate    a 

large  wheel,  which  in 

turn  communicates  its 

motion    through    belts 

to    a.    distant    sawmill 

or  grinder.      In    more 

level      regions      huge 

dams   are    built  which 

hold    back    the    water 

and  keep  it  at  a  higher 

level  than  the  wheel  ; 

from  the  dam  the  water  is  conveyed  in  pipes  (flumes)  to  the 

paddle  wheel  which  it 
turns.  Cogwheels  or  belts 
connect  the  paddle  wheel 
with  the  factory  machinery, 
so  that  motion  of  the  paddle 
wheel  insures  the  running 
of  the  machinery. 

One  of  the  most  efficient 
forms  of   water  wheels  is 
that  shown  in  Figure  121, 
and  called  the  Pelton  wheel. 
FIG.  i2i.—  The  Peiton  water  wheel.          Water   issues  in  a  narrow 

jet    similar  to   that  of  the 

ordinary  garden  hose  and  strikes  with  great  force  against  the 

lower  part  of  the  wheel,  thereby  causing  rotation  of  the  wheel. 

CL.    GEN.    SCI.  —  12 


THE  POWER   BEHIND   THE  ENGINE 


Belts   transfer   this   motion  to  the  machinery  of  factory  or 

mill. 

173.    Turbines.     The  most  efficient  form  of  water  motor  is 

the  turbine,  a  strong  metal  wheel  shaped  somewhat  like  a  pin 

wheel,  inclosed  in  a  heavy  metal  case. 

Water  is  conveyed  from  a  reservoir  or  dam  through  a  pipe 

(penstock)  to  the  turbine  case,  in  which  is  placed  the  heavy 

"metal  turbine  wheel  (Fig.  122).  The  force  of  the  water  causes 
rotation  of  the  turbine  and  of  the  shaft  which 
is  rigidly  fastened  to  it.  The  water  which 
flows  into  the  turbine  case  causes  rotation 
of  the  wheel,  escapes  from  the  case  through 
openings,  and  flows  into  the  tail  water. 

The  power  which  a  turbine  can  furnish 
depends  upon  the  quantity  of  water  and 
the  height  of  the  fall,  and  also  upon  the 
turbine  wheel  itself.  One  of  the  largest 
turbines  known  has  a  horse  power  of  about 
10,000;  that  is,  it  is  equivalent,  approxi- 
mately, to  10,000  horses. 

1 74.  How  much  is  a  Stream  Worth  ? 
The  work  which  a  stream  can  perform 
may  be  easily  calculated.  Suppose,  for 
example,  that  50,000  pounds  of  water  fall 
over  a  22-foot  dam  every  second ;  the  power 
=_^^__r_____.  of  such  a  stream  would  be  1,100,000  foot 
pounds  per  second  or  2000  H.P.  Natur- 

FIG.  122. -A  turbine  at       y         a  t      f   thjg    power    would    be    lost 

Niagara  Falls.  3 ' 

to  use  by  friction  within  the  machinery  and 
by  leakage,  so  that  the  power  of  a  turbine  run  by  a  2000  H.P. 
stream  would  be  less  than  that  value. 

Of  course,  the  horse  power  to  be  obtained  from  a  stream 
determines  the  size  of  the  paddle  wheel  or  turbine  which  can 


I  Water 


HOW  MUCH  IS  A   STREAM   WORTH? 


79 


be  run  by  it.  It  would  be  possible  to  construct  a  turbine  so 
large  that  the  stream  would  not  suffice  to  turn  the  wheel ;  for 
this  reason,  the  power  of  a  stream  is  carefully  determined 
before  machine  construction  is  begun,  and  the  size  of  the 
machinery  depends  upon  the  estimates  of  the  water  power 
furnished  by  expert  engineers. 

A  rough  estimate  of  the  volume  of  a  stream  may  be  made 
by  the  method  described  below  :  — 

Suppose  we  allow  a  stream  of  water  to  flow  through  a 
rectangular  trough ;  the  speed  with  which  the  water  flows 


FIG.  123.  —  Estimating  the  quantity  of  water  which  flows  through  the  trough  each 

second. 

through  the  trough  can  be  determined  by  noting  the  time 
required  for  a- chip  to  float  the  length  of  the  trough;  if  the 
trough  is  10  feet  long  and  the  time  required  is  5  seconds, 
the  water  has  a  velocity  of  2  feet  per  second. 

The  quantity  of  water  which  flows  through  the  trough  each 
second  depends  upon  the  dimensions  of  the  trough  and  the 
velocity  of  the  water.  Suppose  the  trough  is  5  feet  wide  and 
3  feet  high,  or  has  a  cross  section  of  15  square  feet.  If  the 
velocity  of  the  water  were  I  foot  per  second,  then  1 5  cubic  feet 


ISO  THE  POWER   BEHIND    THE  ENGINE 

of  water  would  pass  any  given  point  each  second,  but  since 
the  velocity  of  the  water  is  2  feet  per  second,  30  cubic  feet 
will  represent  the  amount  of  water  which  will  flow  by  a  given 
point  in  one  second. 

175.  Quantity  of  Water  Furnished  by  a  River.    Drive  stakes 
in  the  river  at  various  places  and  note  the  time  required  for 
a  chip  to  float  from  one  stake  to  another.     If  we  know  the 
distance  between  the  stakes  and  the  time  required  for  the  chip 
to  float  from  one  stake  to  another,  the  velocity  of  the  water 
can  be  readily  determined. 

The  width  of  the  stream  from  bank  to  bank  is  easily  meas- 
ured, and  the  depth  is  obtained  in  the  ordinary  way  by  sound- 
ing; it  is  necessary  totake'a  number  of  soundings  because  the 
bed  of  the  river  is  by  no  means  level,  and  soundings  taken  at 
only  one  level  would  not  give  an  accurate  estimate.  If  the 
soundings  show  the  following  depths  .'-30,  25,  20,  32,  28,  the 
average  depth  could  be  taken  as  30  +  25  -f  20  4-  32  +  28  -f- 
5,  or  27  feet.  If,  as  a  result  of  measuring,  the  river  at  a  given 
point  in  its  course  is  found  to  be  27  feet  deep  and  60  feet 
wide,  the  area  of  a  cross  section  at  that  spot  would  be  1620 
square  feet,  and  if  the  velocity  proved  to  be  6  feet  per  second, 
then  the  quantity  of  water  passing  in  any  one  second  would 
be  1620  x  6,  or  9720  cubic  feet.  By  experiment  it  has  been 
found  that  I  cu.  ft.  of  water  weighs  about  62.5  Ib.  The 
weight  of  the  water  passing  each  second  would  therefore  be 
62.5  x  9720,  or  607,500  Ib.  If  this  quantity  of  water  plunges 
over  a  ro-ft.  dam,  it  does  607,500  x  10,  or  6,075,000  foot  pounds 
of  work  per  second,  or  11,045  H.P.  Such  a  stream  would  be 
very  valuable  for  the  running  of  machinery. 

176.  Windmills.     Those  of  us  who  have  spent  our  vacation 
days  in  the  country  know  that  there  is  no  ready-made  water 
supply  there  as  in  the  cities,  but  that  as  a  rule  the  farmhouses 
obtain  their  drinking  water  from  springs  and  wells.    In  poorer 


THE  PRINCIPLE  OF  THE   WINDMILL  181 

houses,  water  is  laboriously  carried  in  buckets  from  the  spring 
or  is  lifted  from  the  well  by  the  windlass.  In  more  prosperous 
houses,  pumps  are  installed ;  this  is  an  improvement  over  the 
original  methods,  but  the  quantity  of  water  consumed  by  the 
average  family  is  so  great  as  to  make  the  task  of  pumping  an 
arduous  one. 

The  average  amount  of  water  used  per  day  by  one  person 
is  25  gallons.  This  includes  water  for  drinking,  cooking,  dish 
washing,  bathing,  laundry.  For  a  family  of  five,  therefore,  the 
daily  consumption  would  be  125  gallons  ;  if  to  this  be  added  the 
water  for  a  single  horse,  cow,  and  pig,  the  total  amount  needed 
will  be  approximately  1 50  gallons  per  day.  A  strong  man  can 
pump  that  amount  from  an  ordinary  well  in  about  one  hour,  but 
if  the  well  is  deep,  m^^e  time  and  strength  are  required. 

The  invention  of  the  windmill  was  a  great  boon  to  country 
folks  because  it  eliminated  from  their  always  busy  life  one 
task  in  which  labor  and  time  were  consumed. 

177.  The  Principle  of  the  Windmill.  The  toy  pin  wheel  is 
a  windmill  in  miniature.  The  wind  strikes  the  sails,  and 
causes  rotation  ;  and  the  stronger 
the  wind  blows,  the  faster  will  the 
wheel  rotate.  In  windmills,  the 
sails  are  of  wood  or  steel,  instead 
of  paper,  but  the  principle  is 
identical. 

As  the  wheel  rotates,  its  motion 
is  communicated  to  a  mechanical 
device  which  makes  use  of  it  to 
raise  and  lower  a  plunger,  and 
hence  as  long  as  the  wind  turns  FlG-  I24-TTfhe  ^.  p!n  .^heel  is  a 

miniature  windmill.     . 

the    windmill,    water    is    raised. 

The  water  thus  raised  empties  into  a  large  tank,  built  either 

in  the  windmill  tower  or  in  the  garret  of  the  house,  and  from 


182 


THE  POWER  BEHIND   THE  ENGINE 


the   tank   the   water   flows    through    pipes   to    the  different 
parts  of  the  house.     On  very  windy  days  the  wheel  rotates 


FIG.  125.  — The  windmill  pumps  water  into  the  tank. 

rapidly,  and  the  tank  fills  quickly ;  in  order  to  guard  against 
an  overflow  from  the  tank  a  mechanical  device  is  installed 
which  stops  rotation  of  the  wheel  when  the  tank  is  nearly 
full.  The  supply  tank  is  usually  large  enough  to  hold  a  supply 
of  water  sufficient  for  several  days,  and  hence  a  continuous 
calm  of  a  day  or  two  does  not  materially  affect  the  house  flow. 
When  once  built,  a  windmill  practically  takes  care  of  itself,  ex- 
cept for  oiling,  and  is  an  efficient  and  cheap  domestic  possession. 
178.  Steam  as  a  Working  Power.  If  a  delicate  vane  is 
held  at  an  opening  from  which  steam  issues,  the  pressure  of 
the  steam  will  cause  rotation  of  the  vane  (Eig.  126),  and  if 
the  vane  is  connected  with  a  machine,  work  can  be  obtained 
from  the  steam. 


STEAM  AS  A    WORKING   POWER 


183 


When  water  is  heated  in  an  open  vessel,  the  pressure  of  its 
steam  is  too  low  to  be  of  practical  value,  but  if  on  the  contrary 


FIG.  126.  —  Steam  as  a  source  of 
power. 


FIG.  127. —  Steam  turbine. 


water  is  heated  in  an  almost  closed  vessel,  its  steam  pressure 
is  considerable.  If  steam  at  high  pressure  is  directed  by  noz- 
zles against  the  blades  of  a  wheel,  rapid  rotation  of  the  wheel 
ensues  just  as  it  did  in  Figure  121,  although  in  this  case  steam 
pressure  replaces  water  pressure.  After  the  steam  has  spent 
itself  in  turning  the  turbine,  it  condenses  into  water  and  makes 
its  escape  through  openings  in  an  inclosing  case.  In  Figure 
127  the  protecting  case  is  removed,  in  order  that  the  form  of 
the  turbine  and  the  positions  of  the  nozzles  may  be  visible. 

The  steam  turbine  is  very  much  more  efficient  than  its 
forerunner,  the  steam  engine.  The  installation  of  turbines 
on  ocean  liners  has  been  accompanied  by  great  increase 
in  speed,  and  by  an  almost  corresponding  decrease  in  the 
cost  of  maintenance. 


1 84 


THE  POWER  BEHIND    THE  ENGINE 


A  single  large  turbine  wheel  may  have  as  many  as  800,000 
sails  or  blades,  and  steam  pours  out  upon  these  from  many 
nozzles. 

179.  Steam  Engines.  A  very  simple  illustration  of  the 
working  of  a  steam  engine  is  given  in  Figure  128.  Steam 

.  under  pressure  enters 
through  the  opening  F, 
passes  through  N,  and 
presses  upon  the  piston  M. 
L  As  a  result  M  moves 
downward,  and  thereby  in- 
duces rotation  in  the  large 
wheel  L. 

As  M  falls  it  drives  the 
air  in  D  out  through  O  and 
P  (the  opening  P  is  not 
visible  in  the  diagram). 

As  soon  as  this  is  accom- 
plished, a  mechanical  de- 
vice draws  up  the  rod  Et 
which  in  turn  closes  the 
opening  N,  and  thus  pre- 
vents the  steam  from  pass- 
ing into  the  part  of  D 
above  M. 

But  when  the  rod  E  is 


FiG.  128.  — The  principle  of  the  steam  engine. 


in  such  a  position  that  N  is  closed,  O  on  the  other  hand  is 
open,  and  steam  rushes  through  it  into  D  and  forces  up  the 
piston.  This  up-and-down  motion  of  the  piston  causes  con- 
tinuous rotation  of  the  wheel  L.  If  the  fire  is  hot,  steam  is 
formed  quickly,  and  the  piston  moves  rapidly ;  if  the  fire  is 
low,  steam  is  formed  slowly,  and  the  piston  moves  less 
rapidly. 


GAS  ENGINES 


I85 


The  steam  engine  as  seen  on  our  railroad  trains  is 
very  complex,  and  cannot  be  discussed  here  ;  in  principle, 
however,  it  is  identical  with  that  just  described.  Fig- 
ure 129  shows  a  steam  harvester  at  work  on  a  modern 
farm. 

In  both  engine  and  turbine  the  real  source  of  power  is 
not  the  steam  but  the  fuel,  such  as  coal  or  oil,  which  converts 
the  water  into  steam. 

1 80.  Gas  Engines.  Automobiles  have  been  largely  re- 
sponsible for  the  gas  engine.  To  carry  coal  for  fuel  and  water 
for  steam  would  be  impracticable  for  most  motor  cars. 
Electricity  is  used  in  some  cars,  but  the  batteries  are  heavy, 
expensive,  and  short-lived,  and  are  not  always  easily  replace- 


FlG.  129.  —  Steam  harvester  at  work. 

able.  For  this  reason  gasoline  is  extensively  used,  and  in  the 
average  automobile  the  source  of  power  is  the  force  generated 
by  exploding  gases. 

It  was  discovered  some  years  ago  that  if  the  vapor  of 
gasoline  or  naphtha  was  mixed  with  a  definite  quantity  of  air, 
and  a  light  was  applied  to  the  mixture,  an  explosion  would 
result.  Modern  science  uses  the  force  of  such  exploding  gases 
for  the  accomplishment  of  work,  such  as  running  of  automo- 
biles and  launches. 


1 86 


THE  POWER   BEHIND    THE  ENGINE 


In  connection  with  the  gasoline  supply  is  a  carburetor  or 
sprayer,  which  sends  into  C  (Fig.  130)  a  fine  mist  of  gaso- 
line vapor  and  air.  When  this  mixture  has  passed  into  C, 
it  is  ignited  by  an  electric  battery  provided  with  an  automatic 
sparking  device.  The  explosion  of  the  gases  is  so  powerful  that 
it  drives  the  piston  P  to  the  right,  and  thus  drives  the  balance 
wheel.  Again,  air  and  gasoline  vapor  enter  C,  the  piston  mean- 
while is  driven  to  the 
left  by  the  balance 
wheel,  and  another 
explosion  occurs. 

The  use  of  naph- 
tha in  launches  is 
familiar  to  many, 


FIG.  130.  —  The  gas  engine. 


and  not  only  are 
launches,  ships,  au- 
tomobiles, making  use  of  gas  power,  but  even  aeroplanes 
are  propelled  in  most  cases  by  the  force  of  exploding  gases. 


CHAPTER    XVIII 

PUMPS   AND   THEIR   VALUE   TO   MAN 

181.  "As  difficult  as  for  water  to  run  up  a  hill!"  Is 
there  any  one  who  has  not  heard  this  saying  ?  And  yet 
most  of  us  accept  as  a  matter  of  course  the  stream  which 
gushes  from  our  faucet,  or  give  no  thought  to  the  ingenuity 
which  devised  a  means  of  forcing  water  upward  through 
pipes.  Despite  the  fact  that  water  flows  naturally  down  hill, 
and  not  up,  we  find  it  available  in  our  homes  and  office 
buildings,  in  some  of  which  it  ascends  to  the  fiftieth  floor ; 
and  we  see  great  streams  of  it  directed  upon  the  tops  of 
burning  buildings  by  firemen  in  the  streets  below. 

In  the  country,  where  there  are  no  great  central  pumping 
stations,  water  for  the  daily  need  must  be  raised  from  wells, 
and  the  supply  of  each  household  is  dependent  upon  the 
labor  anxl  foresight  of  its  members.  The  water  may  be 
brought  to  the  surface  either  by  laboriously  raising  it,  bucket 
by  bucket,  or  by  the  less  arduous  method  of  pumping. 
These  are  the  only  means  possible ;  even  the  windmill  does 
not  eliminate  the  necessity  for  the  pump,  but  merely  replaces 
the  energy  used  by  man  in  working  it. 

In  some  parts  of  our  country  we  have  oil  beds  or  wells. 
But  if  this  underground  oil  is  to  be  of  service  to  man,  it  must 
be  brought  to  the  surface,  and  this  is  accomplished,  as  in  the 
case  of  water,  by  the  use  of  pumps. 

An  old  tin  can  or  a  sponge  may  serve  to  bale  out  water 
from  a  leaking  rowboat,  but  such  a  crude  device  would  be  ab- 

187 


1 88 


PUMPS  AND    THEIR    VALUE   TO   MAN 


surd  if  employed  on  our  huge  vessels  of  war  and  commerce. 
Here  a  rent  in  the  ship's  side  would  mean  inevitable  loss 
were  it  not  possible  to  rid  the  ship  of  the  inflowing  water  by 
the  action  of  strong  pumps. 

Another  and  very  different  use  to  which  pumps  are  put  is 
seen  in  the  compression  of  gases.  Air  is  forced  into  the  tires 
of  bicycles  and  automobiles  until  they  become  sufficiently  in- 
flated to  insure  comfort  in  riding.  Some  present-day  sys- 
tems of  artificial  refrigeration  (Section  93)  could  not  exist 
without  the  aid  of  compressed  gases. 

Compressed  air  has  played  a  very  important  rdle  in  mining, 
being  sent  into  poorly  ventilated  mines  to  improve  the  condi- 
tion of  the  air,  and  to  supply  to  the  miners  the  oxygen  neces- 
sary for  respiration.  Divers  and  men  who  work  under  water 
carry  on  their  backs  a  tank  of  compressed  air,  and  take  from 
it,  at  will,  the  amount  required. 

There  are  many  forms  of  pumps,  and  they  serve  widely 
different  purposes,  being  essential  to  the  operation  of  many 

industrial  undertak- 
ings. In  the  follow- 
ing Sections  some  of 
these  forms  will  be 
studied. 

182.  The  Air  as 
Man's  Servant.  Long 
before  man  harnessed 
water  for  turbines,  or 
steam  for  engines,  he 
made  the  air  serve  his 
purpose,  and  by  means  of  it  raised  water  from  hidden  under- 
ground depths  to  the  surface  of  the  earth;  likewise,  by 
means  of  it,  he  raised  to  his  dwelling  on  the  hillside 
water  from  the  stream  in  the  valley  below.  Those  who 


Fl(~».  131.  — Carrying  water  home  from  the  spring. 


THE   COMMON  PUMP   OR  LIFTING   PUMP 


189 


live  in  cities  where  running  water  is  always  present  in  the 
home  cannot  realize  the  hardship  of  the  days  when  this 
"  ready-made  "  supply  did  not  exist,  but  when  man  labori- 
ously carried  to  his  dwelling,  from  distant  spring  and  stream, 
the  water  necessary  for  the  daily  need. 

What  are  the  characteristics  of  the  air  which  have  enabled 
man  to  accomplish  these  feats  ?  They  are  well  known  to  us 
and  may  be  briefly  stated  as  follows  :  — 

(1)  Air  has  weight,  and  I  cubic  foot  of  air,  at  atmospheric 
pressure,  weighs  \\  ounces. 

(2)  The  air  around  us  presses  with  a  force  of  about  15 
pounds    upon   every  square  inch  of   surface 

that  it  touches. 

(3)  Air  is  elastic;    it  can  be  compressed, 
as  in  the  balloon  or  bicycle  tire,  but  it  ex- 
pands immediately  when  pressure  is  reduced. 
As  it  expands  and  occupies  more  space,  its 
pressure  falls  and  it  exerts  less  force  against 
the    matter  with  which  it  comes  in  contact. 
If,  for  example,  I  cubic  foot  of  air  is  allowed 
to  expand  and  occupy  2  cubic  feet  of  space, 
the  pressure  which  it  exerts  is  reduced  one 
half.     When  air  is  compressed,  its  pressure 
increases,  and  it  exerts  a  greater  force  against 
the  matter  with  which  it  comes  in  contact. 


If    2  cubic  feet  of  air  are  compressed  to   I  FIG.  132.— Theatmos- 

.....  •  '.        phere    pressing 

cubic  foot,  the  pressure  of  the  compressed  air  downward  on  a 
is  doubled.  (See  Section  89.)  ^shes  water  *fter 

v  the  rising  piston  b. 

183.    The  Common  Pump  or  Lifting  Pump. 

Place  a  tube  containing  a  close-fitting  piston  in  a  vessel  of  water, 
as  shown  in  Figure  132.  Then  raise  the  piston  with  the  hand 
and  notice  that  the  water  rises  in  the  piston  tube.  The  rise  of 
water  in  the  piston  tube  is  similar  to  the  raising  of  lemonade 


190  PUMPS  AND   THEIR    VALUE  TO  MAN 

through  a  straw  (Section  77).  The  atmosphere  presses  with 
a  force  of  1 5  pounds  upon  every  square  inch  of  water  in  the 
large  vessel,  and  forces  some  of  it  into  the  space  left  vacant 
by  the  retreating  piston.  The  common  pump  works  in  a  simi- 
lar manner.  It  consists  of  a  piston  or  plunger  which  moves 
back  and  forth  in  an  air-tight  cylinder,  and  contains  an  out- 
ward opening  valve  through  which  water  and  air  can  pass. 
From  the  bottom  of  the  cylinder  a  tube  runs  down  into  the 
well  or  reservoir,  and  water  from  the  well  has  access  to  the 
cylinder  through  another  outward-moving  valve.  In  practice 
the  tube  is  known  as  the  suction  pipe,  and  its  valve  as  the 
suction  valve. 

In  order  to  understand  the  action  of  a  pump,  we  will  sup- 
pose that  no  water  is  in  the  pump,  and  we  will  pump  until 
a  stream  issues  from  the  spout.  The  various  stages  are  repre- 
sented dip-grammatically  by  Figure  133.  In  (i)  the  entire 
pump  is  empty  of  water  but  full  of  air  at  atmospheric  pres- 
sure, and  both  valves  are  closed.  In  (2)  the  plunger  is  be- 
ing raised  and  is  lifting  the  column  of  air  that  rests  on  it. 
The  air  and  water  in  the  inlet  pipe,  being  thus  partially  re- 
lieved of  downward  pressure,  are  pushed  up  by  the  atmospheric 
pressure  on  the  surface  of  the  water  in  the  well.  When 
the  piston  moves  downward  as  in  (3),  the  valve  in  the  pipe 
closes  by  its  own  weight,  and  the  air  in  the  cylinder  escapes 
through  the  valve  in  the  plunger.  In  (4)  the  piston  is  again 
rising,  repeating  the  process  of  (2).  In  (5)  the  process  of  (3) 
is  being  repeated,  but  water  instead  of  air  is  escaping  through 
the  valve  in  the  plunger.  In  (6)  the  process  of  (2)  is  being 
repeated,  but  the  water  has  reached  the  spout  and  is  flowing- 
out. 

After  the  pump  is  in  condition  (6),  motion  of  the  plunger 
is  followed  by  a  more  or  less  regular  discharge  of  water 
through  the  spout,  and  the  quantity  of  water  which  gushes 


THE   COMMON  PUMP  OR  LIFTING  PUMP  191 


J 


J 


5,  piston 


rf,  suction  or 
inlet    pipe 


J 


r,  air- 
tight 
cylinder 


Ji 


J 


a,  suction 
valve 


FIG.  133.  —  Diagram  of  the  process  of  pumping. 


192 


PUMPS  AND    THEIR    VALUE   TO   MAN 


forth  depends  upon  the  speed  with  which  the  piston  is  moved. 
A  strong  man  giving  quick  strokes  can  produce  a  large  flow; 
a   child,  on  the  other  hand,  is  able  to  produce  only  a  thin 
stream.     Whoever  pumps  must  exert  sufficient  force  to  lift 
the  water  from  the  surface  of  the  well  to  the  spout  exit.     For 
this  reason  the  pump  has  received  the  name  of  lifting  primp. 
184.    The  Force  Pump.     In  the  common  pump,  water  can- 
not be  raised   higher  than  the  spout.     In  many  cases  it  is 
desirable  to  force  water  considerably  above 
the  pump  itself,  as,  for  instance,  in  the  fire 
=JL=j|  hose ;  under  such  circumstances  a  type  of 

pump  is  employed  which  has  received  the 
name  of  force  pu mp.  This  differs  but  little 
from  the  ordinary  lift  pump,  as  a  reference 
to  Figure  134  will  show.  Here  both  valves 
are  placed  in  the  cylinder,  and  the  piston 
is  solid,  but  the  principle  is  the  same  as  in 
the  lifting  pump. 

An  upward  motion  of  the  plunger  allows 
water  to  enter  the  cylinder,  and  the  down- 
ward motion  of  the  plunger  drives  water 
through  E.  (Is  this  true  for  the  lift  pump 
as  well  ?)  Since  only  the  downward  motion 
of  the  plunger  forces  water  through  E,  the 
discharge  is  intermittent  and  is  therefore 
not  practical  for  commercial  purposes.  In  order  to  convert  this 
intermittent  discharge  into  a  steady  stream,  an  air  chamber 
is  installed  near  the  discharge  tube,  as  in  Figure  135.  The 
water  forced  into  the  air  chamber  by  the  downward-moving 
piston  compresses  the  air  and  increases  its  pressure.  The 
pressure  of  the  confined  air  reacts  against  the  water  and 
tends  to  drive  it  out  of  the  chamber.  Hence,  even  when  the 
plunger  is  moving  upward,  water  is  forced  through  the  pipe 


FIG.  134.  — Force 
pump. 


IRRIGATION  AND  DRAINAGE 


193 


because  of  the  pressure  of  the  compressed  air.     In  this  way 
a  continuous  flow  is  secured. 

The  height  to  which  the  water  can  be  forced  in  the  pipe  de- 
pends upon  the  size  and  construction  of  the  pump  and  upon  the 
force  with  which  the  plunger  can  be 
moved.  The  larger  the  stream  desired 
and  the  greater  the  height  to  be  reached, 
the  stronger  the  force  needed  and  the 
more  powerful  the  construction  neces- 
sary. 

The  force  pump  gets  its  name  from 
the  fact  that  the  moving  piston  drives  or 
forces  the  water  through  the  discharge 
tube. 

185.  Irrigation  and  Drainage.  History 
shows  that  the  lifting  pump  has  been 
used  by  man  since  the  fourth  century 
before  Christ ;  for  many  present-day 
enterprises  this  ancient  form  of  pump 
is  inconvenient  and  impracticable,  and 
hence  it  has  been  replaced  in  many 
cases  by  more  modern  types,  such  as 
rotary  and  centrifugal  pumps  (Fig.  136).  In  these  forms, 
rapidly  rotating  wheels  lift  the  water  and  drive  it  onward 
into  a  discharge  pipe,  from  which  it  issues  with  great  force. 
There  is  neither  piston  nor  valve  in  these  pumps,  and  the 
quantity  of  water  raised  and  the  force  with  which  it  is  driven 
through  the  pipes  depends  solely  upon  the  size  of  the  wheels 
and  the  speed  with  which  they  rotate. 

Irrigation,  or  the  artificial  watering  of  land,  is  of  the  greatest 
importance  in  those  parts  of  the  world  where  the  land  is  nat- 
urally too  dry  for  farming.     In  the  United  States,  approxi- 
mately two  fifths  of  the  land  area  is  so  dry  as  to  be  worthless 
CL.  GEN.  sci.  — 13 


FIG.  135.  —  The  air  cham- 
ber A  insures  a  continu- 
ous flow  of  water. 


194 


PUMPS  AND   THEIR    VALUE   TO  MA  AT 


for  agricultural  purposes  unless  artificially  watered.  In  the 
West,  several  large  irrigating  systems  have  been  built  by  the 

federal  government, 
and  at  present  about 
ten  million  acres  of 
land  have  been  con- 
verted from  worthless 
farms  into  fields  rich 
in  crops.  Public  irri- 
gating systems  make 
use,  for  the  most  part, 

FIG.  136.  —  Centrifugal  pump  with  part  of  the  casing 

cut  away  to  show  the  wheel.  OI  the  modem  pumps 

to    force   water   over 

long  distances  and  to  supply  it  in  quantities  sufficient  for  vast 
agricultural  needs.  In  many  regions,  the  success  of  a  farm  or 
ranch  depends  upon  the  irrigation  furnished  in  dry  seasons,  or 


FIG.  137.  —  Agriculture  made  possible  by  irrigation. 

upon  man's  ability  to  drive  water  from  a  region  of  abundance 
to  a  remote  region  of  scarcity. 

The  draining  of  land  is  also  a  matter  of  considerable  im- 
portance; swamps  and  marshes  which  were  atone  time  con- 
sidered useless  have  been  reclaimed  and  converted  into  good 
farming  land.  The  drainage  is  usually  done  by  centrifugal 


CAMPING.  — ITS  PLEASURES  AND   ITS  DANGERS     195 

pumps,  since  small  stones  and  foreign  particles  which  would 
clog  the  valves  of  an  ordinary  pump  are  passed  along  with- 
out difficulty  by  the  rotating  shafts. 

1 86.    Camping.  —  Its    Pleasures     and    its    Dangers.     The 
allurement  of  a  vacation  camp  in  the  heart  of  the  woods  is  so 


FIG.  138.  —  Rice  for  its  growth  needs  periodical  flooding,  and  irrigation  often  supplies 
the  necessary  water. 

great  as  to  make  many  campers  ignore  the  vital  importance 
of  securing  a  safe  water  supply.  A  river  bank  may  be 
beautiful  and  teeming  with  diversions,  but  if  the  river  is  used 
as  a  source  of  drinking  water,  the  results  will  almost  always 
be  fatal  to  some.  The  water  can  be  boiled,  it  is  true,  but  'few 
campers  are  willing  to  forage  for  the  additional  wood  needed 
for  this  apparently  unnecessary  requirement ;  then,  too, 
boiled  water  does  not  cool  readily  in  summer,  and  hence  is 
disagreeable  for  drinking  purposes. 

The  only  safe  course  is  to  abandon  the  river  as  a  source 
of  drinking  water,  and  if  a  spring  cannot  be  found,  to  drive 
a  well.  In  many  regions,  especially  in  the  neighborhood  of 


196  PUMPS  AND   THEIR    VALUE  TO  MAN 

streams,  water  can  be  found  ten  or  fifteen  feet  below  the  sur- 
face. Water  taken  from  such  a  depth  has  filtered  through  a 
bed  of  soil,  and  is  fairly  safe  for  any  purpose.  Of  course 
the  deeper  the  well,  the  safer  will  be  the  water.  With  the 
use  of  such  a  pump  as  will  be  described,  campers  can,  without 
grave  danger,  throw  dish  water,  etc.,  on  the  ground  somewhat 
remote  from  the  camp  ;  this  may  not  injure  their  drinking 
water  because  the  liquids  will  slowly  seep  through  the  ground, 
and  as  they  filter  downward  will  lose  their  dangerous  matter. 
All  the  water  which  reaches  the  well  pipes  will  have  filtered 
through  the  soil  bed  and  therefore  will  probably  be  safe. 

But  while  the  careless  disposal  of  wastes  may  not  spoil  the 
drinking  water  (in  the  well  to  be  described),  other  laws  of 
health  demand  a  thoughtful  disposal  of  wastes.  The  mala- 
rial mosquito  and  the  typhoid  fly  flourish  in  unhygienic 
quarters,  and  the  only  way  to  guard  against  their  dangers  is 
to  allow  them  neither  food  nor  breeding  place. 

The  burning  of  garbage,  the  discharge  of  waters  into  cess- 
pools, or,  in  temporary  camps,  the  discharge  of  wastes  to  dis- 
tant points  through  the  agency  of  a  cheap  sewage  pipe 
will  insure  safety  to  campers,  will  lessen  the  trials  of  flies  and 
mosquitoes,  and  will  add  but  little  to  the  expense. 

187.  A  Cheap  Well  for  Campers.  A  two-inch  galvanized 
iron  pipe  with  a  strong,  pointed  end  containing  small  perfora- 
tions is  driven  into  the  ground  with  a  sledge  hammer.  After 
it  has  penetrated  for  a  few  feet,  another  length  is  added  and 
the  whole  is  driven  down,  and  this  is  repeated  until  water  is 
reached.  A  cheap  pump  is  then  attached  to  the  upper  end 
of  the  drill  pipe  and  serves  to  raise  the  water.  During  the 
drilling,  some  soil  particles  get  into  the  pipe  through  the 
perforations,  and  these  cloud  the  water  at  first ;  but  after 
the  pipe  has  once  been  cleaned  by  the  upward-moving  water, 
the  supply  remains  clear.  The  flow  from  such  a  well  is  natur- 


OUR  SUMMER   VACATION 


197 


ally  small ;    first,  because   water  is   not   abundant   near  the 
surface  of  the  earth,  and  second,  because  cheap  pumps  are 
poorly     constructed     and     cannot 
raise  a  large  amount.    But  the  sup- 
ply will   usually   be   sufficient  for 
the  needs  of  simple  camp  life,  and 
many  a  small  farm  uses  this  form 
of   well,    not   only   for   household 
purposes,    but    for    watering    the 
cattle  in  winter. 

If  the  cheapness  of  such  pumps 
were  known,  their  use  would  be 
more  general  for  temporary  pur- 
poses. The  cost  of  material  need 
not  exceed  $5  for  a  ID-foot  well, 
and  the  driving  of  the  pipe  could 
be  made  as  much  a  part  of  the 
camping  as  the  pitching  of  the 
tent  itself.  If  the  camping  site  is 
abandoned  at  the  close  of  the  va- 
cation, the  pump  can  be  removed 
and  kept  over  winter  for  use  the 
following  summer  in  another  place. 
In  this  way  the  actual  cost  of  the 

water  supply  can  be  reduced  to  scarcely  more  than  $3,  the  re- 
movable pump  being  a  permanent  possession.  In  rocky  or 
mountain  regions  the  driven  well  is  not  practicable,  because 
the  driving  point  is  blunted  and  broken  by  the  rock  and  can- 
not pierce  the  rocky  beds  of  land. 

188.  Our  Summer  Vacation.  It  has  been  asserted  by  some 
city  health  officials  that  many  cases  of  typhoid  fever  in  cities 
can  be  traced  to  the  unsanitary  conditions  existing  in  summer 
resorts.  The  drinking  water  of  most  cities  is  now  under 


FIG.  139.  —  A  driven  well. 


I9<S 


PUMPS  AND   THEIR    VALUE  TO  MAN 


strict  supervision,  while  that  of  isolated  farms,  of  small  sea- 
side resorts,  and  of  scattered  mountain  hotels  is  left  to  the 
care  of  individual  proprietors,  and  in  only  too  many  in- 


FIG.  140.  —  Diagram  showing  how  supplying  a  city  with  good  water  lessens 
sickness  and  death.  The  lines  b  show  the  relative  number  of  people  who  died 
of  typhoid  fever  before  the  water  was  filtered ;  the  lines  a  show  the  numbers 
who  died  after  the  water  was  filtered.  The  figures  are  the  number  of  typhoid 
deaths  occurring  yearly  out  of  100,000  inhabitants. 

stances  receives  no  attention  whatever.  The  sewage  disposal 
is  often  inadequate  and  badly  planned,  and  the  water  becomes 
dangerously  contaminated.  A  strong,  healthy  person,  with 
plenty  of  outdoor  exercise  and  with  hygienic  habits,  may  be 
able  to  resist  the  disease  germs  present  in  the  poor  water 
supply  ;  more  often  the  summer  guests  carry  back  with  them 
to  their  winter  homes  the  germs  of  disease,  and  these  gain 
the  upper  hand  under  the  altered  conditions  of  city  and  busi- 
ness life.  It  is  not  too  much  to  say  that  every  man  and 


OUR  SUMMER   VACATION 


199 


woman  should  know  the  source  of  his  summer  table  water 
and  the  method  of  sewage  disposal.  If  the  conditions  are 
unsanitary,  they  cannot  be  reme- 
died at  once,  but  another  resort 
can  be  found  and  personal  danger 
can  be  avoided.  Public  sentiment 
and  the  loss  of  trade  will  go  far 
in  furthering  an  effort  toward 
better  sanitation. 

In  the  driven  well,  water  can- 
not reach  the  spout  unless  it  has 
first  filtered  through  the  soil  to  the 
depth  of  the  driven  pipe ;  after 
such  a  journey  it  is  fairly  safe, 
unless  very  large  quantities  of 
sewage  are  present;  generally 
speaking,  such  a  depth  of  soil  is 
able  to  filter  satisfactorily  the 
drainage  of  the  limited  number 
of  people  which  a  driven  well 
suffices  to  supply. 

Abundant  water  is  rarely 
reached  at  less  than  75  feet,  and 
it  would  usually  be  impossible  to 
drive  a  pipe  to  such  a  depth. 
When  a  large  quantity  of  water 
is  desired,  strong  machines  drill 
into  the  ground  and  excavate  an 
opening  into  which  a  wide  pipe 

,  T       T  FIG.  141.  —  A    deep   well  with    the 

can  be  lowered.    I  recently  spent  «  piston  in  tl£  water. 

a  summer  in  the  Pocono  foun- 
tains and  saw  such  a  well  completed.     The  machine  drilled 
to  a  depth  of  250  feet  before  much  water  was  reached  and 


200 


PUMPS  AND   THEIR    VALUE  TO  MAN 


to  over  300  feet  before  a  flow  was  obtained  sufficient  to  satisfy 
the  owner.  The  water  thus  obtained  was  to  be  the  sole 
water  supply  of  a  hotel  accommodating  150  persons;  the 
proprietor  calculated  that  the  requirements  of  his  guests, 
for  bath,  toilet,  laundry,  kitchen,  etc.,  and  the  domes- 
tics employed  to  serve  them,  together  with  the  livery  at  their 
disposal,  demanded  a  flow  of  10  gallons  per  minute.  The 
ground  was  full  of  rock  and  difficult  to  penetrate,  and  it  re- 
quired 6  weeks  of  constant  work  for  two  skilled  men  to  drill 


FlG.  142.  —  Showing  how  drinking  water  can  be  contaminated  from  cesspool  (c)  and 

wash  water  (w). 

the  opening,  lower  the  suction   pipe,  and  install  the  pump, 
the  cost  being  approximately  $700. 

The  water  from  such  a  well  is  safe  and  pure  except  un- 
der the  conditions  represented  in  Figure  142.  If  sewage 
or  slops  be  poured  upon  the  ground  in  the  neighborhood 
of  the  well,  the  liquid  will  seep  through  the  ground  and 
some  may  make  its  way  into  the  pump  before  it  has  been 
purified  by  the  earth.  The  impure  liquid  will  thus  contami- 
nate the  otherwise  pure  water  and  will  render  it  decidedly 
harmful.  For  absolute  safety  the  sewage  discharge  should 


PUMPS   WHICH  COMPRESS  AIR  20 1 

be  at  least  75  feet  from  the  well,  and  in  large  hotels,  where 
there  is  necessarily  a  large  quantity  of  sewage,  the  distance 
should  be  much  greater.  As  the  sewage  seeps  through  the 
ground  it  loses  its  impurities,  but  the  quantity  of  earth  re- 
quired to  purify  it  depends  upon  its  abundance  ;  a  small 
depth  of  soil  cannot  take  care  of  an  indefinite  amount  of 
sewage.  Hence,  the  greater  the  number  of  people  in  a 
hotel,  or  the  more  abundant  the  sewage,  the  greater  should 
be  the  distance  between  well  and  sewer. 

By  far  the  best  way  to  avoid  contamination  is  to  see  to  it  that 
the  sewage  discharges  into  the  ground  beloiv  the  well ;  that 
is,  to  dig  the  well  in  such  a  location  that  the  sewage  drainage 
will  be  away  from  the  well. 

In  cities  and  towns  and  large  summer  communities,  the 
sewage  of  individual  buildings  drains  into  common  tanks 
erected  at  public  expense ;  the  contents  of  these  are  dis- 
charged in  turn  into  harbors  and  streams,  or  are  otherwise 
disposed  of  at  great  expense,  although  they  contain  valuable 
substances.  It  has  been  estimated  that  the  drainage  or  sewage 
of  England  alone  would  be  worth  $  80,000,000  a  year  if  used 
as  fertilizer. 

A  few  cities,  such  as  Columbus  and  Cleveland,  Ohio,  realize 
the  need  of  utilizing  this  source  of  wealth,  and  by  chemical 
means  deodorize  their  sewage  and  change  it  into  substances 
useful  for  agricultural  and  industrial  purposes.  There  is  still 
a  great  deal  to  be  learned  on  this  subject,  and  it  is  possible 
that  chemically  treated  sewage  may  be  made  a  source  of 
income  to  a  community  rather  than  an  expense. 

189.  Pumps  which  Compress  Air.  The  pumps  considered 
in  the  preceding  Sections  have  their  widest  application  in 
agricultural  districts,  where  by  means  of  them  water  is  raised 
to  the  surface  of  the  earth  or  is  pumped  into  elevated  tanks. 
From  a  commercial  and  industrial  standpoint  a  most  im- 


2O2 


PUMPS  AND   THEIR    VALUE  TO  MAN 


portant  class  of  pump  is  that  known  as  the  compression 
type;  in  these,  air  or  any  other  gas  is  compressed  rather 
than  rarefied. 

Air  brakes  and  self-opening  and  self-closing  doors  on  cars 
are  operated  by  means  of  compression  pumps.  The  laying 
of  bridge  and  pier  foundations,  in  fact  all  work  which  must 
be  done  under  water,  is  possible  only  through  the  agency  of 
compression  pumps.  Those  who  have  visited  mines,  and 
have  gone  into  the  heart  of  the  under-ground  labyrinth, 
know  how  difficult  it  is  for  fresh  air  to  make  its  way  to  the 
miners.  Compression  pumps  have  eliminated  this  difficulty, 
and  to-day  fresh  air  is  constantly  pumped  into  the  mines  to 
supply  the  laborers  there.  Agricultural  methods  also  have 
been  modified  by  the  compression  pump.  The  spraying  of 

trees  (Fig.  143), 
formerly  done 
slowly  and  labo- 
riously, is  now 
a  relatively 
simple  matter. 

190.  The  Bi- 
cycle Pump. 
The  bicy  c  le 
pump  is  the 
best  known  of 
all  compression 
pumps.  Here, 
as  in  other 
pumps  of  its 

type,  the  valves  open  inward  rather  than  outward.  When 
the  piston  is  lowered,  compressed  air  is  driven  through 
the  rubber  tubing,  pushes  open  an  inward-opening  valve 
in  the  tire,  and  thus  enters  the  tire.  When  the  piston  is 


FIG.  143.  —  Spraying  trees  by  means  of  a  compression  pump. 


HOIV  A   MAN  WORKS   UNDER  .WATER 


203 


p 


FIG.  144.  —The  bicycle  foot 
pump. 


raised,  the  lower  valve  closes,  the  upper  valve  is  opened  by 

atmospheric  pressure,  and  air  from  out- 
side enters  the  cylinder;  the  next  stroke 

of  the  piston  drives  a  fresh  supply  of 

air  into  the  tire,  which  thus  in  time  be- 
comes inflated.    In  most  cheap  bicycle 

pumps,  the  piston  valve  is  replaced  by 

a  soft  piece  of  leather  so  attached  to  the 

piston  that  it  allows  air  to  slip  around 

it  and  into  the  cylinder,  but  prevents 

its  escape  from  the  cylinder  (Fig.  144). 
191.   How  a  Man  works  under  Water. 

Place  one  end  of  a  piece  of  glass  tube 

in  a  vessel  of  water  and  notice  that  the 

water   rises    in    the   tube  (Fig.    145). 

Blow   into   the  tube  and   see  whether 

you  can    force  the  water   wholly  or 
|  partially  down  the  tube.     If  the  tube 

is  connected  to  a  small  compression 
pump,  sufficient  air  can  be  sent  into 
the  tube  to  cause  the  water  to  sink 
and  to  keep  the  tube  permanently 
clear  of  water.  This  is,  in  brief,  the 
principle  employed  for  work  under 
water.  A  compression  pump  forces 
air  through  a  tube  into  the  chamber  in 
which  men  are  to  work  (Fig.  146). 
The  air  thus  furnished  from  above  sup- 
plies the  workmen  with  oxygen,  and  by 
its  pressure  prevents  water  from  enter  - 
ing.the  chamber.  When  the  task  has 
been  completed,  the  chamber  is  raised 

and  later  lowered  to  a  new  position. 


FIG.  145.  —  Water  does  not 
enter  the  tube  as  long  as  we 
blow  into  it. 


2O4 


PUMPS  AND   THEIR    VALUE   TO  MAN 


Figure  147  shows  men  at  work  on  a  bridge    foundation. 

Workmen,  tools,  and  sup- 
plies are  lowered  in  baskets 
through  a  central  tube  BC 
provided  with  an  air  chamber 
Z,  having  air-tight  gates  at 
A  and  A'.  The  gate  A'  is 
opened  and  workmen  enter 
the  air  chamber.  The  gate 
A'  is  then  closed  and  the 
gate  A  is  opened  slowly  to 
give  the  men  time  to  get 
accustomed  to  the  high  pres- 
sure in  B,  and  then  the  men 
Excavated  earth  is  removed  in 


FIG.    146.  —  The   principle   of  work   under 
water. 


are  lowered  to  the  bottom, 
a  similar  manner.  Air 
is  supplied  through  a 
tube  DD.  Such  an 
arrangement  for  work 
under  water  is  called  a 
caisson.  It  is  held  in 
position  by  a  mass  of 
concrete  EE. 

In  many  cases  men 
work  in  diving  suits 
rather  than  in  caissons  ; 
these  suits  are  made  of 
rubber  except  for  the 
head  piece,  which  is  of 
metal  provided  with 
transparent  eyepieces. 


FlG.  147.  —  Showing  how  men   can  work  under 
water. 


Air  is  supplied  through 

a  flexible  tube  by  a  compression  pump.    The  diver  sometimes 


COMBINATION  OF  PUMPS  205 

carries  on  his  back  a  tank  of  compressed  air,  from  which  the 
air  escapes  through  a  tube  to  the  space  between  the  body 
and  the  suit.  When  the  air  has  become  foul,  the  diver 
opens  a  valve  in  his  suit  and  allows  it  to  pass  into  the  water, 
at  the  same  time  admitting  a  fresh  supply  from  the  tank. 
The  valve  opens  outward  from  the  body,  and  hence  will  allow 
of  the  exit  of  air  but  not  of  the  entrance  of  water.  When  the 
diver  ceases  work  and  desires  to  rise  to  the  surface,  he  signals 
and  is  drawn  up  by  a  rope  attached  to  the  suit. 

192.  Combination  of  Pumps.  In  many  cases  the  combined 
use  of  both  exhaust  and  compression  pumps  is  necessary  to 
secure  the  desired  result ;  as,  for  example,  in  pneumatic  dis- 
patch tubes.  These  are  employed  in  the  transportation  of 
letters  and  small  packages  from  building  to  building  or  be- 
tween parts  of  the  same  building.  A  pump  removes  air 
from  the  part  of  the  tube  ahead  of  the  package,  and  thus  re- 
duces the  resistance,  while  a  compression  pump  forces  air 
into  the  tube  behind  the  package  and  thus  drives  it  forward 
with  great  speed. 


CHAPTER   XIX 

THE   WATER   PROBLEM   OF  A  LARGE   CITY 

193.  It  is  by  no  means  unusual  for  the  residents  of  a  large 
city  or  town  to  receive  through  the  newspapers  a  notification 
that  the  city  water  supply  is  running  low  and  that  economy 
should  be  exercised  in  its  use.     The  problem  of  supplying  a 
large  city  with  an  abundance  of  pure  water  is   among   the 
most  difficult  tasks  which  city  officials  have  to  perform,  and  is 
one  little  understood  and  appreciated  by  the  average  citizen. 

Intense  interest  in  personal  and  domestic  affairs  is  natural, 
but  every  citizen,  rich  or  poor,  should  have  an  interest  in 
civic  affairs  as  well,  and  there  is  no  better  or  more  important 
place  to  begin  than  with  the  water  supply.  One  of  the 
most  stirring  questions  in  New  York  to-day  has  to  do  with 
the  construction  of  huge  aqueducts  designed  to  convey  to  the 
residents  of  the  city,  water  from  the  distant  Catskill  Mountains. 
The  growth  of  the  population  has  been  so  phenomenally  rapid 
that  the  combined  output  of  all  available  near-by  sources  does 
not  suffice  to  meet  the  increasing  consumption. 

Where  does  your  city  obtain  its  water  ?  Does  it  bring  it  to 
its  reservoirs  in  the  most  economic  way  possible,  and  is  there 
any  legitimate  excuse  for  the  scarcity  of  water  which  many 
communities  face  in  dry  seasons  ? 

194.  Two    Possibilities.      Sometimes    a   city    is   fortunate 
enough  to  be  situated  near  hills  and  mountains  through  which 
streams  flow,  and  in  that  case  the  water  problem  is  simple. 
In  such  a  case  all  that  is  necessary  is  to  run  pipes,  usually 
underground,  from  the  elevated  lakes  or  streams  to  the  in- 

206 


TWO   POSSIBILITIES 


207 


dividual  houses,  or  to  common  reservoirs  from  which  it  is 
distributed  to  the  various  buildings. 

Figure  148  illustrates  in  a  simple  way  the  manner  in  which 
a  mountain  lake  may  serve  to  supply  the  inhabitants  of  a 


FIG.  148.  — The  elevated  mountain  lake  serves  as  a  source  of  water. 

valley.  The  city  of  Denver,  for  example,  is  surrounded  by 
mountains  abounding  in  streams  of  pure,  clear  water ;  pipes 
convey  the  water  from  these  heights  to  the  city,  and  thus  a 
cheap  and  adequate  flow  is  obtained.  Such  a  system  is 
known  as  the  gravity  system.  The  nearer  and  steeper  the 
elevation,  the  greater  the  force  with  which  the  water  flows 
through  the  valley  pipes,  and  hence  the  stronger  the  dis- 
charge from  the  faucets. 

Relatively  few  cities  and  towns  are  so  favorably  situated  as 
regards  water ;  more  often  the  mountains  are  too  distant,  or 
the  elevation  is  too  slight,  to  be  of  practical  value.  Cities 
situated  in  plains  and  remote  from  mountains  are  obliged  to 
utilize  the  water  of  such  streams  as  flow  through  the  land, 
forcing  it  to  the  necessary  height  by  means  of  pumps. 
Streams  which  flow  through  populated  regions  are  apt  to  be 
contaminated,  and  hence  water  from  them  requires  public 


208         THE   WATER   PROBLEM  OF  A   LARGE  CITY 


filtration.     Cities  using  such  a  water  supply  thus  have  the 
double  expense  of  pumping  and  filtration. 

195.  The  Pressure  of  Water.  No  practical  business  man 
would  erect  a  turbine  or  paddle  wheel  without  calculating  in 
advance  the  value  of  his  water  power.  The  paddle  wheel 
might  be  so  heavy  that  the  stream  could  not  turn  it,  or  so 
frail  in  comparison  with  the  water  force  that  the  stream  would 
destroy  it.  In  just  as  careful  a  manner,  the  size  and  the 
strength  of  municipal  reservoirs  and  pumps  must  be  calcu- 
lated. The  greater  the  quantity  of  water  to  be  held  in  the 
reservoir,  the  heavier  are  the  walls  required  ;  the  greater  the 
elevation  of  the  houses,  the  stronger  must  be  the  pumps  and 
the  engines  which  run  them. 

In  order  to  understand  how  these  calculations  are  made, 
we  must  study  the  physical  characteristics  of  water  just  as  we 

studied  the  physical  characteristics 
of  air. 

When  we  measure  water,  we  find 
that  i  cubic  foot  of  it  weighs  about 
62.5  pounds;  this  is  equivalent  to 
saying  that  water  I  foot  deep  presses 
on  the  bottom  of  the  containing 
vessel  with  a  force  of  62.5  pounds 
to  the  square  foot.  If  the  water  is 
2  feet  deep,  the  load  supported  by 
the  vessel  is  doubled,  and  the  pres- 
sure on  each  square  foot  of  the 
bottom  of  the  vessel  will  be  125 
FIG.  149.  — Water  i  foot  deep  pounds,  and  if  the  water  is  10  feet 

deeP>  the  load  b°rne  ^  each  SClUare 

foot  will  be  62  5  pounds.  The  deeper 
the  water,  the  greater  will  be  the  weight  sustained  by  the  con- 
fining vessel  and  the  greater  the  pressure  exerted  by  the  water. 


WHY    WATER  SUPPLY  IS  NOT  UNIFORM 


209 


Since  the  pressure  borne  by  i  square  foot  of  surface  is 
62.5  pounds,  the  pressure  supported  by  I  square  inch  of  sur- 
face is  -jj^  of  62.5  pounds,  or  .43  pound,  nearly  \  pound. 
Suppose  a  vessel  held  water  to  the  depth  of  10  feet,  then 
upon  every  square  inch  of  the  bottom  of  that  vessel  there 
would  be  a  pressure  of  4.34  pounds.  If  a  one-inch  tap 
were  inserted  in  the  bottom  of  the  vessel  so  that  the  water 
flowed  out,  it  would  gush  forth  with  a  force  of  4.34  pounds. 
If  the  water  were  20  feet  deep,  the  force  of  the  outflowing 
water  would  be  twice  as  strong,  because  the  pressure  would 
be  doubled.  But  the 
flow  would  not  remain 
constant,  because  as 
the  water  leaves  the 
outlet,  less  and  less  of 
it  remains  in  the  vessel, 
and  hence  the  pressure 
gradually  sinks  and  the 
flow  drops  correspond- 
ingly.' 

In  seasons  of  pro- 
longed drought,  the 
streams  which  feed  a 
city  reservoir  are  apt 
to  contain  less  than  the 
usual  amount  of  water, 
hence  the  level  of  the 
water  supply  sinks,  the 
pressure  at  the  outlet 
falls,  and  the  force  of 


FlG.  150.  —  The  pressure  at  an  outlet  decreases  as 
the  level  of  the  water  supply  sinks. 


the  outflowing  water  is  lessened  (Fig.  150). 

196.    Why  the  Water  Supply  is  not  uniform  in  All  Parts  of 
the  City.     In    the  preceding  Section,  we  saw  that  the  flow 

CL.    GEN.    SCI.  —  14 


210         THE   WATER  PROBLEM  OF  A   LARGE  CITY 

from  a  faucet  depends  upon  the  height  of  the  reserve  water 
above  the  tap.  Houses  on  a  level  with  the  main  supply  pipes 
(Figs.  148  and  151)  have  a  strong  flow  because  the  water  is 
under  the  pressure  of  a  column  A  ;  houses  situated  on  eleva- 
tion B  have  less  flow,  because  the  water  is  under  the  pres- 
sure of  a  shorter  column  B\  and  houses  at  a  considerable 
elevation  C  have  a  less  rapid  flow  corresponding  to  the 
diminished  depth  (C). 

Not  only  does  the  flow  vary  with  the  elevation  of  the  house, 
but  it  varies  with  the  location  of  the  faucet  within  the  house. 
Unless  the  reservoir  is  very  high,  or  the  pumps  very  power- 
ful, the  flow  on  the  upper  floors  is  noticeably  less  than  that 


FlG.  151.  —  Water  pressure  varies  in  different  parts  of  a  water  system. 

in  the  cellar,  and  in  the  upper  stories  of  some  high  building 
the  flow  is  scarcely  more  than  a  feeble  trickle. 

When  the  respective  flows  at  A,  B,  and  C  (Fig.  151)  are 
measured,  they  are  found  to  be  far  lower  than  the  pressures 
which  columns  of  water  of  the  heights  A,  B,  and  C  have  been 
shown  by  actual  demonstration  to  exert.  This  is  because 
water,  in  flowing  from  place  to  place,  expends  force  in  over- 
coming the  friction  of  the  pipes  and  the  resistance  of  the  air. 
The  greater  the  distance  traversed  by  the  water  in  its  journey 
from  reservoir  to  faucet,  the  greater  the  waste  force  and  the 
less  the  final  flow. 

In  practice,  large  mains  lead  from  the  reservoir  to  the  city, 
smaller  mains  convey  the  water  to  the  various  sections  of  the 
city,  and  service  pipes  lead  to  the  individual  house  taps. 


WHY  WATER  DOES  NOT  ALWAYS  FLOW 


211 


During  this  long  journey,  considerable  force  is  expended 
against  friction,  and  hence  the  flow  at  a  distance  from  the 
reservoir  falls  to  but  a  fraction  of  its  original  strength.  For 
this  reason,  buildings  situated  near  the  main  supply  have  a 
much  stronger  flow  (Fig.  152)  than  those  on  the  same  level 
but  remote  from  the  supply.  Artificial  reservoirs  are  usually 
constructed  on  the  near  outskirts  of  a  town  in  order  that  the 


FIG.  152.  —  The  more  distant  the  fountain,  the  weaker  the  flow. 

frictional  force  lost  in  transmission  may  be  reduced  to  a 
minimum. 

In  the  case  of  a  natural  reservoir,  such  as  an  elevated  lake 
or  stream,  the  distance  cannot  be  planned  or  controlled. 
New  York,  for  example,  will  secure  an  abundance  of  pure 
water  from  the  Catskill  Mountains,  but  it  will  lose  force  in 
transmission.  Los  Angeles  is  undertaking  one  of  the  greatest 
municipal  projects  of  the  day.  Huge  aqueducts  are  being 
built  which  will  convey  pure  mountain  water  a  distance  of 
250  miles,  and  in  quantities  sufficient  to  supply  two  million 
people.  According  to  calculations,  the  force  of  the  water 
will  be  so  great  that  pumps  will  not  be  needed. 

197.  Why  Water  does  not  always  flow  from  a  Faucet. 
Most  of  us  have  at  times  been  annoyed  by  the  inability  to 
secure  water  on  an  upper  story,  because  of  the  drawing  off 


212         THE   WATER  PROBLEM  OF  A  LARGE  CITY 


of  a  supply  on  a  lower  floor.  During  the  working  hours  of 
the  day,  immense  quantities  of  water  are  drawn  off  from  in- 
numerable faucets,  and  hence  the  quantity  in  the  pipes  de- 
creases considerably  unless  the  supply  station  is  able  to  drive 
water  through  the  vast  network  of  pipes  as  fast  as  it  is  drawn 
off.  Buildings  at  a  distance  from  the  reservoir  surfer  under 
such  circumstances,  because  while  the  diminished  pressure  is 
ordinarily  powerful  enough  to  supply  the  lower  floors,  it  is 
frequently  too  weak  to  force  a  continuous  stream  to  high 
levels.  At  night,  however,  and  out  of  working  hours,  few 
faucets  are  open,  less  water  is  drawn  off  at  any  one  time,  and 
the  intricate  pipes  are  constantly  full  of  water  under  high 
pressure.  At  such  times,  a  good  flow  is 
obtainable  even  on  the  uppermost  floors. 

In  order  to  overcome  the  disadvantage 
of  a  decrease  in  flow  during  the  day,  stand- 
pipes  (Fig.  153)  are  sometimes  placed  in 
various  sections.  These  are  practically 
small  steel  reservoirs  full  of  water  and 
connecting  with  the  city  pipes.  During 
"  rush"  hours,  .water  passes  from  these  into 
the  communicating  pipes  and  increases 
the  available  supply,  while  during  the 
night,  when  the  faucets  are  turned  off, 
water  accumulates  in  the  standpipe  against 
the  next  emergency '(Figs.  151  and  154). 
The  service  rendered  by  the  standpipe  is 
similar  to  that  of  the  air  cushion  discussed 
in  Section  184. 
FIG.  153.- A  standpipe.  ^-  The  Cost  of  Water.  In  the  gravity 


system,  where  an  elevated  lake  or  stream 
serves  as  a  natural  reservoir,  the  cost  of  the  city's  waterworks 
is  practically  limited  to  the  laying  of  pipes.  But  when  the 


THE  COST  OF  WATER 


213 


source  of  the  supply  is  more  or  less  on  a  level  with  the  sur- 
rounding land,  the  cost  is  great,  because  the  supply  for 
the  entire  city  must  either  be  pumped  into  an  artificial  reser- 
voir, from  which  it  can  be  distributed,  or  else  must  be  driven 
directly  through  the  mains  (Fig.  154). 

A  gallon  of  water  weighs  approximately  8.3  pounds,  and 
hence  the  work  done  by  a  pump  in  raising  a  gallon  of  water 
to  the  top  of  an  average  house,  an  elevation  of  50  feet,  is 
8-3  X  50,  or  415  foot  pounds.  A  small  manufacturing  town 
uses  at  least  1,000,000  gallons  daily,  and  the  work  done  by  a 


FIG.  154. —  Water  must  be  got  to  the  houses  by  means  of  pumps. 

pump  in  raising  that  amount  to  an  elevation  of  50  feet  would 
be  8.3  x  1,000,000  x  50,  or  415,000,000  foot  pounds. 

The  total  work  done  during  the  day  by  the  pump,  or  the 
engine  driving  the  pump,  is  415,000,000  foot  pounds,  and 
hence  the  work  done  during  one  hour  would  be  ^  of 
415,000,000,  or  17,291,666  foot  pounds  ;  the  work  done  in  one 
minute  would  be  g1^  of  17,291,666,  or  288,194  foot  pounds, 
and  the  work  done  each  second  would  be  g1^  of  288,194,  or 
4803  foot  pounds. 

A  i-H.P.  engine  does  550  foot  pounds  of  work  each 
second,  and  therefore  if  the  pump  is  to  be  operated  by 
an  engine,  the  strength  of  the  latter  would  have  to  be  8.7 
H.P.  An  8.7-H.P.  pumping  engine  working  at  full  speed 
every  second  of  the  day  and  night  would  be  able  to  supply 


214        THE   WATER  PROBLEM  OF  A   LARGE  CITY 

the  town  with  the  necessary  amount  of  water.  When,  how- 
ever, we  consider  the  actual  height  to  which  the  water  is 
raised  above  the  pumping  station,  and  the  extra  pumping 
which  must  be  done  in  order  to  balance  the  frictional  loss, 
it  is  easy  to  understand  that  in  actual  practice  a  much  more 
powerful  engine  would  be  needed.  The  larger  the  piston 
and  the  faster  it  works,  the  greater  is  the  quantity  of  water 
raised  at  each  stroke,  and  the  stronger  must  be  the  engine 
which  operates  the  pump. 

In  many  large  cities  there  is  no  one  single  pumping  station 
from  which  supplies  run  to  all  parts  of  the  city,  but  several 
pumping  stations  are  scattered  throughout  the  city,  and  each 
of  them  supplies  a  restricted  territory. 

199.  The  Bursting  of  Dams  and  Reservoirs.  The  construc- 
tion of  a  safe  reservoir  is  one  of  the  most  important  problems 
of  engineers.  In  October,  1911,  a  town  in  Pennsylvania  was 
virtually  wiped  out  of  existence  because  of  the  bursting  of  a 
dam  whose  structure  was  of  insufficient  strength  to  resist  the 
strain  of  the  vast  quantity  of  water  held  by  it.  A  similar 
breakage  was  the  cause  of  the  fatal  Johnstown  flood  in  1889, 
which  destroyed  no  less  than  seven  towns,  and  in  which  approxi- 
mately 2000  persons  are  said  to  have  lost  their  lives. 

Water  presses  not  only  on  the  bottom  of  a  vessel,  but  upon 
the  sides  as  well ;  a  bucket  leaks  whether  the  hole  is  in  its 
side  or  its  bottom,  showing  that  water  presses  not  only  down- 
ward but  outward.  Usually  a  leak  in  a  dam  or  reservoir 
occurs  near  the  bottom.  Weak  spots  at  the  top  are  rare  and 
easily  repaired,  but  a  leak  near  the  bottom  is  usually  fatal, 
and  in  the  case  of  a  large  reservoir  the  outflowing  water 
carries  death  and  destruction  to  everything  in  its  path. 

If  the  leak  is  near  the  surface,  as  at  a  (Fig.  155),  the  water 
issues  as  a  feeble  stream,  because  the  pressure  against  the 
sides  at  that  level  is  due  solely  to  the  relatively  small  height 


THE  BURSTING    OF  DAMS  AND   RESERVOIRS       21$ 


FIG.  155.  — The  flow  from  an 
opening  depends  upon  the 
height  of  water  above  the  open- 
ing. 


of  water  above  a  (Section  195).     If  the  leak  is  lower,  as  at  b, 

the  issuing  stream  is  stronger  and  swifter,  because  at  that 

level   the  outward  pressure  is  much  greater  than  at  a,  the 

increase  being  due  to  the  fact  that 

the  height   of  the  water  above  b  is 

greater  than  that  above  a.     If  the 

leak  is  quite  low,  as  at  c,  the  issuing 

stream  has  a  still  greater  speed  and 

strength,  and   gushes   forth  with  a 

force  determined  by  the  height   of 

the  water  above  c. 

The  dam  at  Johnstown~was  nearly 
\  mile  wide,  and  40  feet  high,  and  so 
great  was  the  force  and  speed  of 
the  escaping  stream  that  within  an 
hour  after  the  break  had  occurred, 
the  water  had  traveled  a  distance  of  18  miles,  and  had  de- 
stroyed property  to  the  value  of  millions  of  dollars. 

If  a  reservoir  has  a  depth  of  100  feet,  the  pressure  exerted 
upon  each  square  foot  of  its  floor  is  62.5  x  100,  or  6250 
pounds ;  the  weight  therefore  to  be  sustained  by  every 
square  foot  of  the  reservoir  floor  is  somewhat  more  than 
3  tons,  and  hence  strong  foundations  are  essential.  The 
outward  lateral  pressure  at  a  depth  of  25  feet  would  be  only 
one  fourth  as  great  as  that  on  the  bottom  —  hence  the  strain 
on  the  sides  at  that  depth  would  be  relatively  slight,  and  a 
less  powerful  construction  would  suffice.  But  at  a  depth  of 
50  feet  the  pressure  on  the  sides  would  be  one  half  that  of 
the  floor  pressure,  or  i  \  tons.  At  a  depth  of  75  feet,  the 
pressure  on  the  sides  would  be  three  quarters  that  on  the 
bottom,  or  2\  tons.  As  the  bottom  of  the  reservoir  is  ap- 
proached, the  pressure  against  the  sides  increases,  and  more 
powerful  construction  becomes  necessary. 


2l6         THE   WATER   PROBLEM  OF  A   LARGE  CITY 

Small  elevated  tanks,  like  those  of  the  windmill,  frequently 
have  heavy  iron  bands  around  their  lower  portion  as  a  pro- 
tection against  the  extra  strain. 

Before  erecting  a  dam  or  reservoir,  the  maximum  pressure 
to  be  exerted  upon  every  square  inch  of  surface  should  be 

accurately  calculated,  and 
the  structure  should  then  be 
built  in  such  a  way  that  the 
varying  pressure  of  the 
water  can  be  sustained.  It 
is  not  sufficient  that  the 
bottom  be  strong  ;  the  sides 
likewise  must  support  their 
strain,  and  hence  must  be 
FIG.  156.— The  lock  gates  must  be  strong  in  increased  in  strength  with 

order  to  withstand  the  great  pressure  of  the  .  , 

water  against  them.  depth.  This  strengthening 

of  the  walls  is  seen  clearly 

in  the  reservoir  shown  in  Figure  152.  The  bursting  of  dams 
and  reservoirs  has  occasioned  the  loss  of  so  many  lives,  and  the 
destruction  of  so  much  property,  that  some  states  are  consider- 
ing the  advisability  of  federal  inspection  of  all  such  structures. 

200.  The  Relation  of  Forests  to  the  Water  Supply.  When 
heavy  rains  fall  on  a  bare  slope,  or  when  snow  melts  on  a 
barren  hillside,  a  small  amount  of  the  water  sinks  into  the 
ground,  but  by  far  the  greater  part  of  it  runs  off  quickly  and 
swells  brooks  and  streams,  thus  causing  floods  and  freshets. 

When,  however,  rain  falls  on  a  wooded  slope,  the  action  is 
reversed ;  a  small  portion  runs  off,  while  the  greater  portion 
sinks  into  the  soft  earth.  This  is  due  partly  to  the  fact  that 
the  roots  of  trees  by  their  constant  growth  keep  the  soil  loose 
and  open,  and  form  channels,  as  it  were,  along  which  the 
water  can  easily  run.  It  is  due  also  to  the  presence  on  the 
ground  of  decaying  leaves  and  twigs,  or  humus.  The  decay- 


RELATION  OF  FORESTS   TO    THE   WATER   SUPPLY    217 

ing  vegetable  matter  which  covers  the  forest  floor  acts  more 
or  less  as  a  sponge,  and  quickly  absorbs  falling  rain  and 
melting  snow.  The  water  which  thus  passes  into  the  humus 
and  the  soil  beneath  does  not  remain  there,  but  slowly  seeps 
downward,  and  finally  after  weeks  and  months  emerges  at  a 
lower  level  as  a  stream.  Brooks  and  springs  formed  in  this 
way  are  constant  feeders  of  rivers  and  lakes. 

In  regions  where  the  land  has  been  deforested,  the,  rivers 
run  low  in  season  of  prolonged  drought,  because  the  water 
which  should  have  slowly  seeped  through  the  soil,  and  then 
supplied  the  rivers  for  weeks  and  months,  ran  off  from  the 
barren  slopes  in  a  few  days. 

Forests  not  only  lessen  the  danger  of  floods,  but  they  con- 
serve our  waterways,  preventing  a  dangerous  high-water 
mark  in  the  season  of  heavy  rains  and  melting  snows,  and 
then  preventing  a  shrinkage  in  dry  seasons  when  the  only 
feeders  of  the  rivers  are  the  underground  sources.  In  the 
summer  of  1911,  prolonged  drought  in  North  Carolina  low- 
ered the  rivers  to  such  an  extent  that  towns  dependent  upon 
them  suffered  greatly.  The  city  of  Charlotte  was  reduced 
for  a  time  to  a  practically  empty  reservoir;  washing  and 
bathing  were  eliminated,  machinery  dependent  upon  water- 
power  and  steam  stood  idle,  and  every  glass  of  water  drunk 
was  carefully  reckoned.  Thousands  of  gallons  of  water  were 
brought  in  tanks  from  neighboring  cities,  and  were  emptied 
into  the  empty  reservoir  from  whence  it  -trickled  slowly 
through  the  city  mains.  The  lack  of  water  caused  not  only 
personal  inconvenience  and  business  paralysis,  but  it  occa- 
sioned real  danger  of  disease  through  unflushed  sewers  and 
insufficiently  drained  pipes. 

The  conservation  of  the  forest  means  the  conservation  of 
our  waterways,  whether  these  be  used  for  transportation  or 
as  sources  of  drinking  water. 


CHAPTER   XX 

MAN'S   CONQUEST  OF  SUBSTANCES 

201.  Chemistry.  Man's  mechanical  inventions  have  been 
equaled  by  his  chemical  researches  and  discoveries,  and  by 
the  application  he  has  made  of  his  new  knowledge. 

The  plain  cotton  frock  of  our  grandmothers  had  its 
death  knell  sounded  a  few  years  ago,  when  John  Mercer 
showed  that  cotton  fabrics  soaked  in  caustic  soda  assumed 
under  certain  conditions  a  silky  sheen,  and  when  dyed  took 
on  beautiful  and  varied  hues.  The  demonstration  of  this 
simple  fact  laid  the  foundation  for  the  manufacture  of  a  vast 
variety  of  attractive  dress  materials  known  as  mercerized 
cotton. 

Possibly  no  industry  has  been  more  affected  by  chemical 
discovery  than  that  of  dyeing.  Those  of  us  who  have  seen 
the  old  masterpieces  in  painting,  or  reproductions  of  them, 
know  the  softness,  the  mellowness,  the  richness  of  tints  em- 
ployed by  the  old  masters.  But  if  we  look  for  the  brilliancy 
and  variety  of  color  seen  in  our  owji  day,  the  search  will  be 
fruitless,  because  these  were  unknown  until  a  half  century 
ago.  Up  to  that  time,  dyes  were  few  in  number  and  were 
extracted  solely  from  plants,  principally  from  the  indigo  and 
madder  plants.  But  about  the  year  1856  it  was  discovered 
that  dyes  in  much  greater  variety  and  in  purer  form  could 
be  obtained  from  coal  tar.  This  chemical  production  of  dyes 
has  now  largely  supplanted  the  original  method,  and  the  in- 
dustry has  grown  so  rapidly  that  a  single  firm  produced  in 

218 


CHEMISTRY 

one  year  from  coal  tar  a  quantity  of  indigo  dye  which  under 
the  natural  process  of  plant  extraction  would  have  required 
a  quarter  million  acres  of  indigo  plant. 

The  abundance  and  cheapness  of  newspapers,  coarse 
wrapping  papers,  etc.,  is  due  to  the  fact  that  man  has  learned 
to  substitute  wood  for  rags  in  the  manufacture  of  paper.  In- 
vestigation brought  out  the  fact  that  wood  contained  the  sub- 
stance which  made  rags  valuable  for  paper  making.  Since 
the  supply  of  rags  was  far  less  than  the  demand,  the  problem 
of  the  extraction  from  wood  of  the  paper-forming  substance 
was  a  vital  one.  From  repeated  trials,  it  was  found  that 
caustic  soda  when  heated  with  wood  chips  destroyed  every- 
thing in  the  wood  except  the  desired  substance,  cellulose; 
this  could  be  removed,  bleached,  dried,  and  pressed  into 
paper.  The  substitution  of  wood  for  rags  has  made  possible 
the  daily  issue  of  newspapers,  for  the  making  of  which  suffi- 
cient material  would  not  otherwise  have  been  available. 
When  we  reflect  that  a  daily  paper  of  wide  circulation  con- 
sumes ten  acres  of  wood  lot  per  day,  we  see  that  all  the 
rags  in  the  world  would  be  inadequate  to  meet  this  demand 
alone,  to  say  nothing  of  periodicals,  books,  tissue  paper,  etc. 

Chemistry  plays  a  part  in  every  phase  of  life ;  in  the  arts, 
the  industries,  the  household,  and  in  the  body  itself,  where 
digestion,  excretion,  etc.,  result  from  the  action  of  the  bodily 
fluids  upon  food.  The  chemical  substances  of  most  interest 
to  us  are  those  which  affect  us  personally  rather  than  indus- 
trially; for  example,  soap,  which  cleanses  our  bodies,  our 
clothing,  our  household  possessions;  washing  soda,  which 
lightens  laundry  work ;  lye,  which  clears  out  the  drain  pipe 
clogged  with  grease ;  benzine,  which  removes  stains  from 
clothing ;  turpentine,  which  rids  us  of  paint  spots  left  by 
careless  workmen ;  and  hydrogen  peroxide,  which  disinfects 
wounds  and  sores. 


220  MAWS  CONQUEST  OF  SUBSTANCES 

In  order  to  understand  the  action  of  several  of  these  sub- 
stances we  must  study  the  properties  of  two  groups  of  chemi- 
cals —  known  respectively  as  acids  and  bases ;  the  first  of 
these  may  be  represented  by*  vinegar,  sulphuric  acid,  and 
oxalic  acid ;  and  the  second,  by  ammonia,  lye,  and  limewater. 

202.  Acids.     All  of  us  know  that  vinegar  and  lemon  juice 
have  a  sour  taste,  and  it  is  easy  to  show  that  most  acids  are 
characterized  by  a  sour  taste.     If  a  clean  glass  rod  is  dipped 
into  very  dilute  acid,  such  as  acetic,  sulphuric,  or  nitric  acid, 
and  then  lightly  touched  to  the  tongue,  it  will  taste  sour.     But 
the  best  test  of  an  acid  is  by  sight  rather  than  by  taste,  be- 
cause it  has  been  found  that  an  acid  is  able  to  discolor  a 
plant  substance  called  litmus.     If  paper  is  soaked  in  a  litmus 
solution  until  it  acquires  the  characteristic  blue  hue  of  the 
plant  substance,  and  is  then  dried  thoroughly,  it  can  be  used 
to  detect  acids,  because  if  it  comes  in  contact  with  even  the 
minutest  trace  of   acid,  it  loses  its  blue  color  and  assumes 
a  red  tint.     Hence,  in  order  to  detect  the  presence  of  acid  in 
a  substance,  one  has  merely  to  put  some  of  the   substance 
on   blue  litmus  paper,   and  note  whether  or  not  the  latter 
changes  color.     This  test  shows  that  many  of  our  common 
foods  contain  some  acid ;    for  example,  fruit,  buttermilk,  sour 
bread,  and  vinegar. 

The  damage  which  can  be  done  by  strong  acids  is  well 
known ;  if  a  jar  of  sulphuric  acid  is  overturned,  and  some  of 
it  falls  on  the  skin,  it  eats  its  way  into  the  flesh  and  leaves 
an  ugly  sore ;  if  it  falls  on  carpet  or  coat,  it  eats  its  way 
into  the  material  and  leaves  an  unsightly  hole.  The  evil  re- 
sults of  an  accident  with  acid  can  be  lessened  if  we  know 
just  what  to  do  and  do  it  quickly,  but  for  this  we  must  have 
a  knowledge  of  bases,  the  second  group  of  chemicals. 

203.  Bases.     Substances  belonging  to  this  group  usually 
have  a  bitter  taste  and  a  slimy,  soapy  feeling.  .  For  our  present 


BASES  221 

purposes,  the  most  important  characteristic  of  a  base  is  that 
it  will  neutralize  an  acid  and  in  some  measure  hinder  the 
damage  effected  by  the  former.  If,  as  soon  as  an  acid  has 
been  spilled  on  cloth,  a  base,  such  as  ammonia,  is  applied  to 
the  affected  region,  but  little  harm  will  be  done.  In  your 
laboratory  experiments  you  may  be  unfortunate  enough  to 
spill  acid  on  your  body  or  clothing ;  if  so,  quickly  apply 
ammonia.  If  you  delay,  the  acid  does  its  work,  and  there  is 
no  remedy.  If  soda  (a  base)  touches  black  material,  it  dis- 
colors it  and  leaves  an  ugly  brown  spot ;  but  the  application 
of  a  little  acid,  such  as  vinegar  or  lemon  juice,  will  often 
restore  the  original  color  and  counteract  the  bad  effects  of 
the  base.  Limewater  prescribed  by  physicians  in  cases  of  ill- 
ness is  a  well-known  base.  This  liquid  neutralizes  the  too 
abundant  acids  present  in  a  weak  system  and  so  quiets  and 
tones  the  stomach. 

The  interaction  of  acids  and  bases  may  be  observed  in 
another  way.  If  blue  litmus  paper  is  put  into  an  acid  solu- 
tion, its  color  changes  to  red  ;  if  now  the  red  litmus  paper  is 
dipped  into  a  base  solution,  caustic  soda,  for  example,  its 
original  color  is  partially  restored.  What  the  acid  does,  the 
base  undoes,  either  wholly  or  in  part.  Bases  always  turn  red 
litmus  paper  blue. 

Bases,  like  acids,  are  good  or  bad  according  to  their  use;  if 
they  come  in  contact  with  cloth,  they  eat  or  discolor  it,  unless 
neutralized  by  an  acid.  But  this  property  of  bases,  harmful 
in  one  way,  is  put  to  advantage  in  the  home,  where  grease  is 
removed  from  drainpipe  and  sink  by  the  application  of  lye, 
a  strong  base.  If  the  lye  is  too  concentrated,  it  will  not  only 
eat  the  grease,  but  will  corrode  the  metal  piping ;  it  is  easy, 
however,  to  dilute  base  solutions  to  such  a  degree  that  they 
will  not  affect  piping,  but  will  remove  grease.  Dilute  am- 
monia is  used  in  almost  every  home  and  is  an  indispensable 


222  MAWS   CONQUEST  OF  SUBSTANCES 

domestic  servant ;  diluted  sufficiently,  it  is  invaluable  in  the 
washing  of  delicate  fabrics  and  in  the  removing  of  stains,  and 
in  a  more  concentrated  form  it  is  helpful  as  a  smelling  salt 
in  cases  of  fainting. 

Some  concentrated  bases  are  so  powerful  in  their  action  on 
grease,  cloth,  and  metal  that  they  have  received  the  designa- 
tion caustic,  and  are  ordinarily  known  as  caustic  soda,  caustic 
potash  (lye),  and  caustic  lime.  These  more  active  bases  are 
generally  called  alkalies  in  distinction  from  the  less  active  ones. 

204.  Neutral  Substances.  To  any  acid  solution  add  grad- 
ually a  small  quantity  of  a  base,  and  test  the  mixture  from 
time  to  time  with  blue  litmus  paper ;  at  first  the  paper  will  turn 
red  quickly,  but  as  more  and  more  of  the  base  is  added  to  the 
solution,  it  has  less  and  less  effect  on  the  blue  litmus  paper, 
and  finally  a  point  is  reached  when  a  fresh  strip  of  blue  paper 
will  not  be  affected.  Such  a  result  indicates  infallibly  the 
absence  of  any  acid  qualities  in  the  solution.  If  now  red  litmus 
paper  is  tested  in  the  same  solution,  its  color  also  will  remain 
unchanged ;  such  a  result  indicates  infallibly  the  absence  of 
any  basic  quality.  The  solution  has  the  characteristic  property 
of  neither  acid  nor  base  and  is  said  to  be  neutral. 

If  to  the  neutral  solution  an  extra  portion  of  base  is  added, 
so  that  there  is  an  excess  of  base  over  acid,  the  neutralization 
is  overbalanced  and  the  red  paper  turns  blue.  If  to  the 
neutral  solution  an  extra  portion  of  acid  is  added,  so  that 
there  is  an  excess  of  acid  over  base,  the  neutralization  is 
overbalanced  in  the  opposite  direction,  and  the  solution  ac- 
quires acid  characteristics. 

Most  acids  and  bases  will  eat  and  corrode  and  discolor,  while 
neutral  substances  will  not ;  it  is  for  this  reason  that  soap,  a 
slightly  alkaline  substance,  is  the  safest  cleansing  agent  for 
laundry,  bath,  and  general  work.  Good  soaps,  being  care- 
fully made,  are  so  nearly  neutral  that  they  will  not  fade  the 


SOAP 


223 


color  out  of  clothing ;  the  cheap  soaps  are  less  carefully  pre- 
pared and  are  apt  to  have  a  strong  excess  of  the  base  ingred- 
ient ;  such  soaps  are  not  safe  for  delicate  work. 

205.  Soap.  If  we  gather  together  scrapings  of  lard,  butter, 
bits  of  tallow  from  burned-out  candles,  scraps  of  waste  fat,  or 
any  other  sort  of  grease,  and  pour  a  strong  solution  of  lye  over 
the  mass,  a  soft  soapy  substance  is  formed.  In  colonial 
times,  every  family  made  its  own  supply  of  soap,  utilizing, 
for  that  purpose,  household  scraps  often  regarded  by  the 
housekeeper  of  to-day  as  worthless.  Grease  and  fat  were 
boiled  with  water  and  hardwood  ashes,  which  are  rich  in  lye, 
and  from  the  mixture  came  the  soft  soap  used  by  our  an- 
cestors. In  practice,  the  wood  ashes  were  boiled  in  water, 
which  was  then  strained  off,  and  the  resulting  filtrate,  or  lye, 
was  mixed  with  the  fats  for  soap  making. 

Most  fats  contain  a  substance  of  an 
acid  nature,  and  are  decomposed  by  the 
action  of  bases  such  as  caustic  soda  and 
caustic  potash.  The  acid  component  of 
the  grease  partially  neutralizes  the  base, 
and  a  new  substance  is  formed,  namely, 
soap. 

With  the  advance  of  civilization  the 
labor  of  soap  making  passed  from  the 
home  to  the  factory,  very  much  as  bread 
making  has  done  in  our  own  day.  Dif- 
ferent varieties  of  soaps  appeared,  of 
which  the  hard  soap  was  the  most  popu- 
lar, owing  to  the  ease  with  which  it  could 
be  transported.  Within  the  last  few 
years  liquid  soaps  have  come  into  favor,  especially  in  schools, 
railroad  stations,  and  other  public  places,  where  a  cake  of 
soap  would  be  handled  by  many  persons.  By  means  of  a 


FIG.    157.  —  Liquid    soap 
container. 


224  MAN^S   CONQUEST  OF  SUBSTANCES 

simple  device  (Fig.  157),  the  soap  escapes  from  a  receptacle 
when  needed.  The  mass  of  the  soap  does  not  come  in  con- 
tact with  the  skin,  and  hence  the  spread  of  contagious  skin 
diseases  is  lessened. 

Commercial  soaps  are  made  from  a  great  variety  of  sub- 
stances, such  as  tallow,  lard,  castor  oil,  coconut  oil,  olive  oil, 
etc.;  or  in  cheaper  soaps,  from  rosin,  co.ttonseed  oil,  and 
waste  grease.  The  fats  which  go  to  waste  in  our  garbage 
could  be  made  a  source  of  income,  not  only  to  the  housewife, 
but  to  the  city.  In  Columbus,  Ohio,  garbage  is  used  as  a 
source  of  revenue ;  the  grease  from  the  garbage  being  sold 
for  soap  making,  and  the  tankage  (Section  188)  for  fertilizer. 

206.  Why    Soap    Cleans.     The   natural   oil   of    the   skin 
catches  and  retains  dust  and  dirt,  and  makes  a  greasy  film 
over  the  body.     This  cannot  be  removed  by  water  alone,  but 
if  soap  is  used  and  a  generous  lather  is  applied  to  the  skin, 
the  dirt  is  "cut"  and  passes  from  the  body  into  the  water. 
Soap  affects  a  grease  film  and  water  very  much  as  the  white  of 
an  egg  affects  oil  and  water.     These  two  liquids  alone  do  not 
mix,  the  oil  remaining  separate  on  the  surface  of  the  water ; 
but  if  a  small  quantity  of  white  of  egg  .is  added,  an  emulsion  is 
formed,  the  oil  separating  into  minute  droplets  which  spread 
through  the  water.     In  the  same  way,  soap  acts  on  a  grease 
film,  separating  it  into  minute  droplets  which  leave  the  skin  and 
spread  through  the  water,  carrying  with  them  the  dust  and  dirt 

\  particles.     The  warmer  the  water,  the  better  will  be  the  emul- 

1  sion,  and  hence  the  more  effective  the  removal  of  dirt  and 

grease.      This   explanation   holds   true   for   the  removal   of 

grease  from    any   surface,   whether  of   the   body,    clothing, 

furniture,  or  dishes. 

207.  Washing  Powders.     Sometimes  soap  refuses  to  form 
a  lather  and  instead  cakes  and  floats  as  a  scum  on  the  top 
of  the  water;    this  is  not  the  fault  of  the  soap  but  of  the 


WASHING  POWDERS  22$ 

water.  As  water  seeps  through  the  soil  or  flows  over  the  land, 
it  absorbs  and  retains  various  soil  constituents  which  modify 
its  character  and,  in  some  cases,  render  it  almost  useless  for 
household  purposes.  Most  of  us  are  familiar  with  the  rain 
barrel  of  the  country  house,  and  know  that  the  housewife 
prefers  rain  water  for  laundry  and  general  work.  Rain 
water,  coming  as  it  does  from  the  clouds,  is  free  from  the 
chemicals  gathered  by  ground  water,  and  is  hence  practically 
pure.  While  foreign  substances  do  not  necessarily  injure 
water  for  drinking  purposes  (Section  69),  they  are  often  of  such 
a  nature  as  to  prevent  soap  from  forming  an  emulsion,  and 
hence  from  doing  its  work.  Under  such  circumstances  the 
water  is  said  to  be  hard,  and  soap  used  with  it  is  wasted. 
Even  if  water  is  only  moderately  hard,  much  soap  is  lost 
The  substances  which  make  water  hard  are  calcium  and  mag--' 
nesium  salts.  When  soap  is  put  into  water  containing  one  or 
both  of  these,  it  combines  with  the  salts  to  form  sticky  insolu- 
ble scum.  It  is  therefore  not  free  to  form  an  emulsion  and  to 
remove  grease.  As  a  cleansing  agent  it  is  valueless.  The 
average  city  supply  contains  so  little  hardness  that  it  is  satis- 
factory for  toilet  purposes ;  but  in  the  laundry,  where  there  is 
need  for  the  full  effect  of  the  soap,  and  where  the  slightest 
loss  would  aggregate  a  great  deal  in  the  course  of  time,  some- 
thing must  be  done  to  counteract  the  hardness.  The  addition 
of  soda,  or  calcium  carbonate  to  the  water  will  usually  produce 
the  desired  effect..  Washing  soda  combines  with  calcium  and 
magnesium  and  prevents  them  from  uniting  with  soap.  The 
soap  is  thus  free  to  form  an  emulsion,  just  as  in  ordinary 
Water.  Washing  powders  are  sometimes  used  instead  of  wash- 
ing soda.  Most  washing  powders  contain,  in  addition  to  a 
softening  agent,  some  alkali,  and  hence  a  double  good  is 
obtained  from  their  use ;  they  not  only  soften  the  water  and 
allow  the  soap  to  form  an  emulsion,  but  they  also,  through 

CL.    GEN.    SCI. —  15 


226  MAWS  CONQUEST  OF  SUBSTANCES 

their  alkali  content,  cut  the  grease  and  themselves  act  as 
cleansers.  In  some  cities  where  the  water  is  very  hard,  as 
in  Columbus,  Ohio,  it  is  softened  and  filtered  at  public  ex- 
pense, before  it  leaves  the  reservoirs.  But  even  under 
these  circumstances,  a  moderate  use  of  washing  powder  is 
general  in  laundry  work. 

If  washing  powder  is  put  on  clothes  dry,  or  is  thrown  into 
a  crowded  tub,  it  will  eat  the  clothes  before  it  has  a  chance 
to  dissolve  in  the  water.  The  only  safe  method  is  to  dissolve 
the  powder  before  the  clothes  are  put  into  the  tub.  The 
trouble  with  our  public  laundries  is  that  many  of  them  are 
careless  about  this  very  fact,  and  do  not  take  time  to  dissolve 
the  powder  before  mixing  it  with  the  clothes. 

The  strongest  washing  powder  is  soda,  and  this  cheap  form 
is  as  good  as  any  of  the  more  expensive  preparations  sold 
under  fancy  names.  Borax  is  a  milder  powder  and  is  desir- 
able for  finer  work. 

One  of  the  most  disagreeable  consequences  of  the  use  of 
hard  water  for  bathing  is  the  unavoidable  scum  which  forms 
on  the  sides  of  bathtub  and  washbowl.  The  removal  of  the 
caked  grease  is  difficult,  and  if  soap  alone  is  used,  the  cleaning 
of  the  tub  requires  both  patience  and  hard  scrubbing.  The 
labor  can  be  greatly  lessened  by  moistening  the  scrubbing 
cloth  with  turpentine  and  applying  it  to  the  greasy  film,  which 
immediately  dissolves  and  thus  can  be  easily  removed.  The 
presence  of  the  scum  can  be  largely  avoided  by  adding  a 
small  amount  of  liquid  ammonia  to  the  bath  water.  But 
many  persons  object  to  this;  hence  it  is  well  to  have  some 
other  easy  method  of  removing  the  objectionable  matter. 

208.  To  remove  Stains  from  Cloth.  While  soap  is,  gener- 
ally speaking,  the  best  cleansing  agent,  there  are  occasions 
when  other  substances  can  be  used  to  better  advantage.  For 
example,  grease  spots  on  carpet  and  non-washable  dress  goods 


SALTS  227 

are  best  removed  by  the  application  of  gasoline  or  benzine.  |; 
These  substances  dissolve  the  grease,  but  do  not  remove  it 
from  the  clothing;  for  that  purpose  a  woolen  cloth  should  be 
laid  under  the  stain  in  readiness  to  absorb  the  benzine  and  the 
grease  dissolved  in  it.  If  the  grease  is  not  absorbed  while  in 
solution,  it  remains  in  the  clothing  and  after  the  evaporation 
of  the  benzine  reappears  in  full  force. 

Cleaners  frequently  clean  suits  by  laying  a  blotter  over  a 
grease  spot  and  applying  a  hot  iron  ;  the  grease,  when  melted 
by  the  heat,  takes  the  easiest  way  of  spreading  itself  and 
passes  from  cloth  to  blotter. 

209.  Salts.  A  neutral  liquid  formed  as  in  Section  204,  by 
the  action  of  hydrothloric  acid  and  the  alkali  solution  of  caus- 
tic soda,  has  a  brackish,  salty  taste,  and  is,  in  fact,  a  solution 
of  salt.  This  can  be  demonstrated  by  evaporating  the 
neutral  liquid  to  dryness  and  examining  the  residue  of  solid 
matter,  which  proves  to  be  common  salt. 

When  an  acid  is  mixed  with  a  base,  the  result  is  a  sub- 
stance more  or  less  similar  in  its  properties  to  common  salt; 
for  this  reason  all  compounds  formed  by  the  neutralization  of 
an  acid  with  a  base  are  called  salts.  If,  instead  of  hydro- 
chloric acid  (HC1),  we  use  an  acid  solution  of  potassium 
tartrate,  and  if  instead  of  caustic  soda  we  use  bicarbonate 
of  soda  (baking  soda),  the  result  is  a  brackish  liquid  as  be- 
fore, but  the  salt  in  the  liquid  is  not  common  salt,  but  Rochelle 
salt.  Different  combinations  of  acids  and  bases  produce 
different  salts.  Of  all  the  vast  group  of  salts,  the  most 
abundant  as  well  as  the  most  important  is  common  salt, 
known  technically  as  sodium  chloride  because  of  its  two  con- 
stituents, sodium  and  chlorine. 

We  are  not  dependent  upon  neutralization  for  the  enor- 
mous quantities  of  salt  used  in  the  home  and  in  commerce. 
It  is  from  the  active,  restless  seas  of  the  present,  and  from 


228  MAWS  CONQUEST  OF  SUBSTANCES 

the  dead  seas  of  the  prehistoric  past  that  our  vast  stores  of 
salt  come.  The  waters  of  the  Mediterranean  and  of  our  own 
Great  Salt  Lake  are  led  into  shallow  basins,  where,  after 
evaporation  by  the  heat  of  the  sun,  they  leave  a  residue  of 
salt.  By  far  the  largest  quantity  of  salt,  however,  comes 
from  the  seas  which  no  longer  exist,  but  which  in  far  remote 
ages  dried  up  and  left  behind  them  their  burden  of  salt. 
Deposits  of  salt  formed  in  this  way  are  found  scattered 
throughout  the  world,  and  in  our  own  country  are  found  in 
greatest  abundance  in  New  York.  The  largest  salt  deposit 
known  has  a  depth  of  one  mile  and  exists  in  Germany. 

Salt  is  indispensable  on  our  table  and  in  our  kitchen,  but 
the  amount  of  salt  used  in  this  way  is  far  too  small  to  account 
for  a  yearly  consumption  of  4,000,000  tons  in  the  United 
States  alone.  The  manufacture  of  soap,  glass,  bleaching 
powders,  baking  powders,  washing  soda,  and  other  chemicals 
depends  on  salt,  and  it  is  for  these  that  the  salt  beds  are  mined. 

210.  Baking  Soda.  Salt  is  by  all  odds  the  most  important 
sodium  compound.  Next  to  it  come  the  so-called  carbon- 
ates :  first,  sodium  carbonate,  which  is  already  familiar  to  us 
as  washing  soda ;  and  second,  sodium  bicarbonate,  which  is 
an  ingredient  of  baking  powders.  These  are  both  obtained 
from  sodium  chloride  by  relatively  simple  means ;  that  is,  by 
treating  salt  with  the  base,  ammonia,  and  with  carbon  dioxide. 

Washing  soda  has  already  been  discussed.  Since  baking 
powders  in  some  form  are  used  in  almost  all  homes  for  the 
raising  of  cake  and  pastry  dough,  it  is  essential  that  their 
helpful  and  harmful  qualities  be  clearly  understood. 

The  raising  of  dough  by  means  of  baking  soda  —  bicarbon- 
ate of  soda  —  is  a  very  simple  process.  When  soda  is  heated, 
it  gives  off  carbon  dioxide  gas ;  you  can  easily  prove  this  for 
yourself  by  burning  a  little  soda  in  a  test  tube,  and  testing 
the  escaping  gas  in  a  test  tube  of  limewater.  When  flour  and 


BAKING  POWDERS  2 2Q 

water  alone  are  kneaded  and  baked  in  loaves,  the  result  is  a 
mass  so  compact  and  hard  that  human  teeth  are  almost 
powerless  to  crush  and  chew  it.  The  problem  is  to  separate 
the  mass  of  dough  or,  in  other  words,  to  cause  it  to  rise  and 
lighten.  This  can  be  done  by  mixing  a  little  soda  in  the 
flour,  because  the  heat  of  the  oven  causes  the  soda  to  give  off 
bubbles  of  gas,  and  these  in  expanding  make  the  heavy  mass 
slightly  porous.  Bread  is  never  lightened  with  soda  because 
the  amount  of  gas  thus  given  off  is  too  small  to  convert 
heavy  compact  bread  dough  into  a  spongy  mass ;  but  biscuit 
and  cake,  being  by  nature  less  compact  and  heavy,  are  suf- 
ficiently lightened  by  the  gas  given  off  from  soda. 

Buj:  there  is  one  great  objection  to  the  use  of  soda  alone 
as  a  leavening  agent.  After  baking  soda  has  lost  its  carbon 
dioxide  gas,  it  is  no  longer  baking  soda,  but  is  transformed 
into  its  relative,  washing  soda,  which  has  a  disagreeable  taste 
and  is  by  no  means  desirable  for  the  stomach. 

Man's  knowledge  of  chemicals  and  their  effect  on  each 
other  has  enabled  him  to  overcome  this  difficulty  and,  at  the 
same  time,  to  retain  the  leavening  effect  of  the  baking  soda. 

211.  Baking  Powders.  If  some  cooking  soda  is  put  into 
lemon  juice  or  vinegar,  or  any  acid,  bubbles  of  gas  immedi- 
ately form  and  escape  from  the  liquid.  After  the  efferves- 
cence has  ceased,  a  taste  of  the  liquid  will  show  you  that  the 
lemon  juice  has  lost  its  acid  nature,  and  has  acquired  in 
exchange  a  salty  taste.  Baking  soda,  when  treated  with  an 
acid,  is  transformed  into  carbon  dioxide  and  a  salt.  The  various 
baking  powders  on  the  market  to-day  consist  of  baking  soda 
and  some  acid  substance,  which  acts  upon  the  soda,  forces  it 
to  give  up  its  gas,  and  at  the  same  time  unites  with  the  resi- 
due to  form  a  harmless  salt. 

Cream  of  tartar  contains  sufficient  acid  to  act  on  baking 
soda,  and  is  a  convenient  and  safe  ingredient  for  baking  pow- 


230  MAWS   CONQUEST  OF  SUBSTANCES 

der.  When  soda  and  cream  of  tartar  are  mixed  dry,  they  do 
not  react  on  each  other,  neither  do  they  combine  rapidly  in 
cold  moist  dough,  but  as  soon  as  the  heat  of  the  oven  pene- 
trates the  doughy  mass,  the  cream  of  tartar  combines  with 
the  soda  and  sets  free  the  gas  needed  to  raise  the  dough. 
The  gas  expands  with  the  heat  of  the  oven,  raising  the  dough 
still  more.  Meanwhile,  the  dough  itself  is  influenced  by  the 
heat  and  is  stiffened  to  such  an  extent  that  it  retains  its  in- 
flated shape  and  spongy  nature. 

Many  housewives  look  askance  at  ready-made  baking  pow- 
ders and  prefer  to  bake  with  soda  and  sour  milk,  soda  and 
buttermilk,  or  soda  and  cream  of  tartar.  Sour  milk  and  butter- 
milk are  quite  as  good  as  cream  of  tartar,  because  the  lactic 
acid  which  they  contain  combines  with  the  soda  and  liber- 
ates carbon  dioxide,  and  forms  a  harmless  residue  in  the 
dough. 

The  desire  of  manufacturers  to  produce  cheaper  baking 
powders  led  to  the  use  of  cheap  acids  and  alkalies,  regardless 
of  the  character  of  the  resulting  salt.  Alum  and  soda  were 
popular  for  some  time ;  but  careful  examination  proved  that 
the  particular  salt  produced  by  this  combination  was  not 
readily  absorbed  by  the  stomach,  and  that  its  retention  there 
was  injurious  to  health.  For  this  reason,  many  states  have 
prohibited  the  use  of  alum  in  baking  powders. 

It  is  not  only  important  to  choose  the  ingredients  carefully ; 
it  is  also  necessary  to  calculate  the  respective  quantities  of 
each,  otherwise  there  will  be  an  excess  of  acid  or  alkali  for 
the  stomach  to  take  care  of.  A  standard  powder  contains 
twice  as  much  cream  of  tartar  as  of  bicarbonate  of  soda,  and 
the  thrifty  housewife  who  wishes  to  economize,  can  make  for 
herself,  at  small  cost,  as  good  a  baking  powder  as  any  on  the 
market,  by  mixing  tartar  and  soda  in  the  above  proportions 
and  adding  a  little  corn  starch  to  keep  the  mixture  dry. 


SODA   MINTS  231 

The  self-raising  flour,  so  widely  advertised  by  grocers,  is 
flour  in  which  these  ingredients  or  their  equivalent  have  been 
mixed. 

212.  Soda  Mints.  Bicarbonate  of  soda  is  practically  the  sole 
ingredient  of  the  soda  mints  popularly  sold  for  indigestion. 
These  correct  a  tendency  to  sour  stomach  because  they 
counteract  the  surplus  acid  in  the  stomach,  and  form  with  it 
a  safe  neutral  substance. 

Seidlitz  powder  is  a  simple  remedy  consisting  of  two  pow- 
ders, one  containing  bicarbonate  of  soda,  and  the  other,  some 
acid  such  as  cream  of  tartar.  When  these  substances  are 
dissolved  in  water  and  mixed,  effervescence  occurs,  carbon 
dioxide  escapes,  and  a  solution  of  Rochelle  salt  remains. 


CHAPTER    XXI 

FERMENTATION 

213.  While  baking  powder  is  universally  used  for  biscuits 
and  cake,  it  is  seldom,  if  ever,  used  for  bread,  because  it 
does  not  furnish  sufficient  gas  to  lighten  the  tough  heavy 
mass  of  bread  dough.  Then,  too,  most  people  prefer  the 
taste  of  yeast-raised  bread.  There  is  a  reason  for  this  wide- 
spread preference,  but  to  understand  it,  we  must  go  some- 
what far  afield,  and  must  study  not  only  the  bread  of  to-day, 
but  the  bread  of  antiquity,  and  the  wines  as  well. 

If  grapes  are  crushed,  they  yield  a  liquid  which  tastes 
like  the  grapes  ;  but  if  the  liquid  is  allowed  to  stand  in  a  warm 
place,  it  loses  its  original  character,  and  begins  to  ferment, 
becoming,  in  the  course  of  a  few  weeks,  a  strongly  intoxicat- 
ing drink.  This  is  true  not  only  of  grape  juice  but  also  of 
the  juice  of  all  other  sweet  fruits;  apple  juice  ferments  to 
cider,  currant  juice  to  currant  wine,  etc.  This  phenomenon 
of  fermentation  is  known  to  practically  all  races  of  men,  and 
there  is  scarcely  a  savage  tribe  without  some  kind  of  fer- 
mented drink ;  in  the  tropics  the  fermented  juice  of  the  palm 
tree  serves  for  wine ;  in  the  desert  regions,  the  fermented 
juice  of  the  century  plant ;  and  in  still  other  regions,  the  root 
of  the  ginger  plant  is  pressed  into  service. 

The  fermentation  which  occurs  in  bread  making  is  similar 
to  that  which  is  responsible  for  the  transformation  of  plant 
juices  into  intoxicating  drinks.  The  former  process  is  not  so 

232 


THE  BREAD   OF  ANTIQUITY  233 

old,  however,  since  the  use  of  alcoholic  beverages  dates  back 
to  the  very  dawn  of  history,  and  the  authentic  record  of 
raised  or  leavened  bread  is  but  little  more  than  3000  years 
old. 

214.  The  Bread  of  Antiquity.  The  original  method  of 
bread  making  and  the  method  employed  by  savage  tribes  of 
to-day  is  to  mix  crushed  grain  and  water  until  a  paste  is 
formed,  and  then  to  bake  this  over  a  camp  fire.  The  result 
is  a  hard  compact  substance  known  as  unleavened  bread. 
A  considerable  improvement  over  this  tasteless  mass  is 
self-raised  bread.  If  dough  is  left  standing  in  a  warm  place 
a  number  of  hours,  it  swells  up  with  gas  and  becomes  porous, 
and  when  baked,  is  less  compact  and  hard  than  the  savage 
bread.  Exposure  to  air  and  warmth  brings  about  changes  in 
dough  as  well  as  in  fruit  juices,  and  alters  the  character  of 
the  dough  and  the  bread  made  from  it.  Bread  made  in  this 
way  would  not  seem  palatable  to  civilized  man  of  the  present 
day,  accustomed,  as  he  is,  to  delicious  bread  made  light  and 
porous  by  yeast ;  but  to  the  ancients,  the  least  softening  and 
lightening  was  welcome,  and  self-fermented  bread,  therefore, 
supplanted  the  original  unleavened  bread. 

Soon  it  was  discovered  that  a  pinch  of  this  fermented  dough 
acted  as  a  starter  on  a  fresh  batch  of  dough.  Hence,  a  little 
of  the  fermented  dough  was  carefully  saved  from  a  batch, 
and  when  the  next  bread  was  made,  the  fermented  dough,  or 
leaven,  was  worked  into  the  fresh  dough  and  served  to  raise 
the  mass  more  quickly  and  effectively  than  mere  exposure  to 
air  and  warmth  could  do  in  the  same  length  of  time.  This 
use  of  leaven  for  raising  bread  has  been  practiced  for  ages. 

Grape  juice  mixed  with  millet  ferments  quickly  and 
strongly,  and  the  Romans  learned  to  use  this  mixture  for 
bread  raising,  kneading  a  very  small  amount  of  it  through 
the  dough. 


234  FERMENT  A  TION 

215.  The  Cause  of  Fermentation.  Although  alcoholic 
fermentation,  and  the  fermentation  which  goes  on  in  raising 
dough,  were  known  and  utilized  for  many  years,  the  cause  of 
the  phenomenon  was  a  sealed  book  until  the  nineteenth 
century.  About  that  time  it  was  discovered,  through  the 
use  of  the  microscope,  that  fermenting  liquids  contain  an 
army  of  minute  plant  organisms  which  not  only  live  there, 
but  which  actually  grow  and  multiply  within  the  liquid. 
For  growth  and  multiplication,  food  is  necessary,  and  this  the 
tiny  plants  get  in  abundance  from  the  fruit  juices ;  they  feed 
upon  the  sugary  matter  and  as  they  feed,  they  ferment  it, 
changing  it  into  carbon  dioxide  and  alcohol.  The  carbon  di- 
oxide, in  the  form  of  small  bubbles,  passes  off  from  the  fer- 
menting mass,  while  the  alcohol  remains  in  the  liquid,  giving 
the  stimulating  effect  desired  by  imbibers  of  alcoholic  drinks. 
The  unknown  strange  organisms  were  called  yeast,  and  they 
were  the  starting  point  of  the  yeast  cakes  and  .yeast  brews 
manufactured  to-day  on  a  large  scale,  not  only  for  bread 
making  but  for  the  commercial  production  of  beer,  ale,  por- 
ter, and  other  intoxicating  drinks. 

The  grains,  rye,  corn,  rice,  wheat,  from  which  meal  is 
made,  contain  only  a  small  quantity  of  sugar,  but,  on  the 
other  hand,  they  contain  a  large  quantity  of  starch  which  is 
easily  convertible  into  sugar.  Upon  this  the  tiny  yeast 
plants  in  the  dough  feed,  and,  as  in  the  case  of  the  wines, 
ferment  the  sugar,  producing  carbon  dioxide  and  alcohol. 
The  dough  is  thick  and  sticky  and  the  gas  bubbles  expand 
it  into  a  spongy  mass.  The  tiny  yeast  plants  multiply  and 
continue  to  make  alcohol  and  gas,  and  in  consequence,  the 
dough  becomes  lighter  and  lighter.  When  it  has  risen  suf- 
ficiently, it  is  kneaded  and  placed  in  an  oven;  the  heat  of  the 
oven  soon  kills  the  yeast  plants  and  drives  the  alcohol  out  of 
the  bread ;  at  the  same  time  it  expands  the  imprisoned  gas 


WHERE  DOES   YEAST  COME  FROM?  235 

bubbles  and  causes  them  to  lighten  and  swell  the  bread  still 
more.  Meanwhile,  the  dough  has  become  stiff  enough  to  sup- 
port itself.  The  result  of  the  fermentation  is  a  light,  spongy 
loaf. 

216.  Where  does  Yeast  come  From  ?  The  microscopic 
plants  which  we  call  yeast  are  widely  distributed  in  the  air,  and 
float  around  there  until  chance  brings  them  in  contact  with  a 
substance  favorable  to  their  growth,  such  as  fruit  juices  and 
moist  warm  batter.  Under  the  favorable  conditions  of  abun- 
dant moisture,  heat,  and  food,  they  grow  and  multiply  rapidly, 
and  cause  the  phenomenon  of  fermentation.  Wild  yeast 
settles  on  the  skin  of  grapes  and  apples,  but  since  it  does  not 
have  access  to  the  fruit  juices  within,  it  remains  inactive 
very  much  as  a  seed  does  before  it  is  planted.  But  when  the 
fruit  is  crushed,  the  yeast  plants  get  into  the  juice,  and  feed- 
ing  on  it,  grow  and  multiply.  The  stray  yeast  plants  which 
get  into  the  sirup  are  relatively  few,  and  hence  fermentation 
is  slow  ;  it  requires  several  weeks  for  currant  wine  to  ferment, 
and  several  months  for  the  juice  of  grapes  to  be  converted 
into  wine. 

Stray  yeast  finds  a  favorable  soil  for  growth  in  the  warmth 
and  moisture  of  a  batter ;  but  although  the  number  of  these 
stray  plants  is  very  large,  it  is  insufficient  to  cause  rapid 
fermentation,  and  if  we  depended  upon  wild  yeast  for  bread 
raising,  the  result  would  not  be  to  our  liking. 

When  our  remote  ancestors  saved  a  pinch  of  dough  as 
leaven  for  the  next  baking,  they  were  actually  cultivating 
yeast,  although  they  did  not  know  it.  The  reserved  portion 
served  as  a  favorable  breeding  place  to  the  yeast  plants  within 
it ;  they  grew  and  reproduced  amazingly,  and  became  so 
numerous,  that  the  small  mass  of  old  dough  in  which  they 
were  gathered  served  to  leaven  the  entire  batch  at  the  next 
baking. 


236  FERMENT  A  T/Otf 

As  soon  as  man  learned  that  yeast  plants  caused  fermenta- 
tion in  liquors  and  bread,  he  realized  that  it  would  be  to  his 
advantage  to  cultivate  yeast  and  to  add  it  to  bread  and  to 
plant  juices  rather  than  to  depend  upon  accidental  and  slow 
fermentation  from  wild  yeast.  Shortly  after  the  discovery  of 
yeast  in  the  nineteenth  century,  man  commenced  his  attempt 
to  cultivate  the  tiny  organisms.  Their  microscopic  size  added 
greatly  to  his  trouble,  and  it  was  only  after  years  of  careful 
and  tedious  investigation  that  he  was  able  to  perfect  the 
commercial  yeast  cakes  and  yeast  brews  universally  used  by 
bakers  and  brewers.  The  well-known  compressed  yeast  cake 
is  simply  a  mass  of  live  and  vigorous  yeast  plants,  embedded  in 
a  soft,  soggy  material,  and  ready  to  grow  and  multiply  as  soon 
as  they  are  placed  under  proper  conditions  of  heat,  moisture, 
and  food.  Seeds  which  remain  on  our  shelves  do  not  germi- 
nate, but  those  which  are  planted  in  the  soil  do ;  so  it  is  with 
the  yeast  plants.  While  in  the  cake  they  are  as  lifeless  as 
the  seed  ;  when  placed  in  dough,  or  fruit  juice,  or  grain  water, 
they  grow  and  multiply  and  cause  fermentation. 


CHAPTER   XXII 

BLEACHING 

217.  The  beauty  and  the  commercial  value  of  uncolored 
fabrics  depend  upon  the  purity  and  perfection  of  their  white- 
ness ;  a  man's  white  collar  and  a  woman's  white  waist  must 
be  pure  white,  without  the  slightest  tinge  of  color.  But  all 
natural  fabrics,  whether  they  come  from  plants,  like  cotton 
and  linen,  or  from  animals,  like  wool  and  silk,  contain  more 
or  less  coloring  matter,  which  impairs  the  whiteness.  This 
coloring  not  only  detracts  from  the  appearance  of  fabrics 
which  are  to  be  worn  uncolored,  but  it  seriously  interferes  with 
the  action  of  dyes,  and  at  times  plays  the  dyer  strange  tricks. 

Natural  fibers,  moreover,  are  difficult  to  spin  and  weave 
unless  some  softening  material  such  as  wax  or  resin  is  rubbed 
lightly  over  them.  The  matter  added  to  facilitate  spinning 
and  weaving  generally  detracts  from  the.  appearance  of  the 
uncolored  fabric,  and  also  interferes  with  successful  dyeing. 
Thus  it  is  easy  to  see  that  the  natural  coloring  matter  and  the 
added  foreign  matter  must  be  entirely  removed  from  fabrics 
destined  for  commercial  use.  Exceptions  to  this  general  fact 
are  sometimes  made,  because  unbleached  material  is  cheaper 
and  more  durable  than  the  bleached  product,  and  for  some 
purposes  is  entirely  satisfactory ;  unbleached  cheesecloth  and 
sheeting  are  frequently  purchased  in  place  of  the  more 
expensive  bleached  material.  Formerly,  the  only  bleaching 
agent  known  was  the  sun's  rays,  and  linen  and  cotton  were 

237 


238 


BLEACHING 


put  out  to  sun  for  a  week;  that  is,  the  unbleached  fabrics 
were  spread  on  the  grass  and  exposed  to  the  bleaching  action 
of  sun  and  dew. 

218.  An  Artificial  Bleaching  Agent.  While  the  sun's  rays 
are  effective  as  a  bleaching  agent,  the  process  is  slow;  more- 
over, it  would  be  impossible  to  expose  to  the  sun's  rays  the 
vast  quantity  of  fabrics  used  in  the  civilized  world  of  to-day, 
and  the  huge  and  numerous  bolts  of  material  which  daily 
come  from  our  looms  and  factories  must  therefore  be  whitened 
by  artificial  means.  The  substance  almost  universally  used  as  a 
rapid  artificial  bleaching  agent  is  chlorine,  best  known  to  us  as  a 
constituent  of  common  salt.  Chlorine  is  never  free  in  nature, 

but  is  found  in  com- 
bination with  other 
substances,  as,  for 
example,  in  combina- 
tion with  sodium  in 
salt,  or  with  hydrogen 
in  hydrochloric  acid. 

The  best  laboratory 
method  of  securing 
free  chlorine  is  to 
heat  in  a  water  bath  a 
mixture  of  hydrochlo- 
ric acid  and  manganese 
dioxide,  a  compound 
containing  one  part  of 
manganese  and  two 
parts  of  oxygen.  The 
heat  causes  the  man- 
ganese dioxide  to  give 


FIG.  158.  —  Preparing   chlorine  from   hydrochloric 
and  manganese  dioxide. 


up  its  oxygen,  which  immediately  combines  with  the  hydro- 
gen  of  the  hydrochloric  acid  and  forms  water.     The   man- 


BLEACHING  POWDER  239 

ganese  itself  combines  with  part  of  the  chlorine  originally 
in  the  acid;  but  not  with  all.  There  is  thus  some  free  chlo- 
rine left  over  from  the  acid,  and  this  passes  off  as  a  gas 
and  can  be  collected,  as  in  Figure  158.  Free  chlorine  is 
heavier  than  air,  and  hence  when  it  leaves  the  exit  tube  it 
settles  at  the  bottom  of  the  jar,  displacing  the  air,  and  finally 
filling  the  bottle. 

Chlorine  is  a  very  active  substance  and  combines  readily 
with  most  substances,  but  especially  with  hydrogen ;  if 
chlorine  comes  in  contact  with  steam,  it  abstracts  the  hydro- 
gen and  unites  with  it  to  form  hydrochloric  acid,  but  it  leaves 
the  oxygen  free  and  uncombined.  This  tendency  of  chlorine 
to  combine  with  hydrogen  makes  it  valuable  as  a  bleaching 
agent.  In  order  to  test  the  efficiency  of  chlorine  as  a  bleach- 
ing agent,  drop  a  wet  piece  of  colored  gingham  or  calico  into 
the  bottle  of  chlorine,  and  notice  the  rapid  disappearance  of 
color  from  the  sample.  If  unbleached  muslin  is  used,  the 
moist  strip  loses  its  natural  yellowish  hue  and  becomes  a 
clear,  pure  white.  The  explanation  of  the  bleaching  power 
of  chlorine  is  that  the  chlorine  combines  with  the  hydrogen 
of  the  water  and  sets  oxygen  free ;  the  uncombined  free 
oxygen  oxidizes  the  coloring  matter  in  the  cloth  and  destroys  it. 

Chlorine  has  no  effect  on  dry  material,  as  may  be  seen  if 
we  put  dry  gingham  into  the  jar;  in  this  case  there  is  no 
water  to  furnish  hydrogen  for  combination  with  the  chlorine, 
and  no  oxygen  to  be  set  free. 

219.  Bleaching  Powder.  Chlorine  gas  has  a  very  injurious 
effect  on  the  human  body,  and  hence  cannot  be  used  directly 
as  a  bleaching  agent.  It  attacks  the  mucous  membrane 
of  the  nose  and  lungs,  and  produces  the  effect  of  a  severe 
cold  or  catarrh,  and  when  inhaled,  causes  death.  But  certain 
compounds  of  chlorine  are  harmless,  and  can  be  used  instead 
of  chlorine  for  destroying  either  natural  or  artificial  dyes. 


240  BLEACHING 

One  of  these  compounds,  namely,  chloride  of  lime,  is  the 
almost  universal  bleaching  agent  of  commerce.  It  comes  in 
the  form  of  powder,  which  can  be  dissolved  in  water  to  form 
the  bleaching  solution  in  which  the  colored  fabrics  are  im- 
mersed. But  fabrics  immersed  in  a  bleaching  powder  solution 
do  not  lose  their  color  as  would  naturally  be  expected.  The 
reason  for  this  is  that  the  chlorine  gas  is  not  free  to  do  its 
work,  but  is  restricted  by  its  combination  with  the  other  sub- 
stances. By  experiment  it  has  been  found  that  the  addition 
to  the  bleaching  solution  of  an  acid,  such  as  vinegar  or  lemon 
juice  or  sulphuric  acid,  causes  the  liberation  of  the  chlorine. 
The  chlorine  thus  set  free  reacts  with  the  water  and  liberates 
oxygen  ;  this  in  turn  destroys  the  coloring  matter  in  the  fibers, 
and  transforms  the  material  into  a  bleached  product. 

The  acid  used  to  liberate  the  chlorine  from  the  bleaching 
powder,  and  the  chlorine  also,  rot  materials  with  which  they 
remain  in  contact  for  any  length  of  time.  For  this  reason, 
fabrics  should  be  removed  from  the  bleaching  solution  as 
soon  as  possible,  and  should  then  be  rinsed  in  some  solution, 
such  as  ammonia,  which  is  capable  of  neutralizing  the  harm- 
ful substances ;  finally  the  fabric  should  be  thoroughly  rinsed 
in  water  in  order  that  all  foreign  matter  may  be  removed. 
The  reason  home  bleaching  is  so  seldom  satisfactory  is  that 
most  amateurs  fail  to  realize  the  necessity  of  immediate 
neutralization  and  rinsing,  and  allow  the  fabric  to  regain  too 
long  in  the  bleaching  solution,  and  allow  it  to  dry  with  traces 
of  the  bleaching  substances  present  in  the  fibers.  Material 
treated  in  this  way  is  thoroughly  bleached,  but  is  at  the  same 
time  rotten  and  worthless.  Chloride  of  lime  is  frequently 
used  in  laundry  work ;  the  clothes  are  whiter  than  when 
cleaned  with  soap  and  simple  washing  powders,  but  they 
soon  wear  out  unless  the  precaution  has  been  taken  to  add  an 
"  antichlor  "  or  neutralizer  to  the  bleaching  solution. 


WOOL  AND  SILK  BLEACHING 


241 


220.  Commercial  Bleaching.  In  commercial  bleaching  the 
material  to  be  bleached  is  first  moistened  with  a  very  weak 
solution  of  sulphuric  acid  or  hydrochloric  acid,  and  is  then 
immersed  in  the  bleactiing  powder  solution.  As  the  moist 
material  is  drawn  through  the  bleaching  solution,  the  acid  on 
the  fabric  acts  upon  the  solution  and  releases  chlorine.  The 
chlorine  thus  set  free  immediately  attacks  the  coloring  matter 
and  destroys  it,  leaving  the  material  in  a  bleached  condition. 
The  bleached  material  is  then  immersed  in  a  neutralizing 
bath  and  is  finally  rinsed  thoroughly  in  water.  Strips  of 
cotton  or  linen  many  miles  long  are  drawn  by  machinery  into 
and  out  of  the  various  solutions  (Fig.  159),  are  then  passed 


FlG.  159.  —  The  material  to  be  bleached  is  drawn  through  an  acid  a,  then  through  a 
bleaching  solution  b,  and  finally  through  a  neutralizing  solution  c. 

over  pressing  rollers,  and  emerge  snow  white,  ready  to  be 
dyed  or  to  be  used  as  white  fabric.    . 

221.  Wool  and  Silk  Bleaching.  Animal  fibers  like  silk,  wool, 
and  feathers,  and  some  vegetable  fibers  like  straw,  cannot  be 
bleached  by  means  of  chlorine,  because  it  attacks  not  only  the 
coloring  matter  but  the  fiber  itself,  and  leaves  it  shrunken 
and  inferior.  Cotton  and  linen  fibers,  apart  from  the  small 
amount  of  coloring  matter  present  in  them,  contain  nothing 
but  carbon,  oxygen,  and  hydrogen,  while  animal  fibers  con- 
tain in  addition  to  these  elements  some  compounds  of  nitro- 
gen. The  presence  of  these  nitrogen  compounds  influences 
the  action  of  the  chlorine  and  produces  unsatisfactory  results. 
For  animal  fibers  it  is  therefore  necessary  to  discard  chlorine 
CL.  GEN.  sci.  —  1 6 


242  BLEACHING 

as  a  bleaching  agent,  and  to  substitute  a  substance  which  will 
have  a  less  disastrous  action  upon  the  fibers.  Such  a  sub- 
stance is  to  be  had  in  sulphurous  acid.  When  sulphur  burns, 
as  in  a  match,  it  gives  off  disagreeable  fumes,  and  if  these 
are  made  to  bubble  into  a  vessel  containing  water,  they  dis- 
solve and  form  with  the  water  a  substance  known  as  sul- 
phurous acid.  That  this  solution  has  bleaching  properties  is 
shown  by  the  fact  that  a  colored  cloth  dipped  into  it  loses  its 
color,  and  unbleached  fabrics  immersed  in  it  are  whitened. 
The  harmless  nature  of  sulphurous  acid  makes  it  very  desir- 
able as  a  bleaching  agent,  especially  in  the  home. 

Silk,  lace,  and  wool  when  bleached  with  chlorine  become 
hard  and  brittle,  but  when  whitened  with  sulphurous  acid, 
they  retain  their  natural  characteristics. 

This  mild  form  of  a  bleaching  substance  has  been  put  to 
uses  which  are  now  prohibited  by  the  pure  food  laws.     In 
some  canneries  common  corn  is  whitened  with  sulphurous 
acid,  and  is  then  sold  under  false  representations.     Cherries 
are  sometimes  bleached  and  then  colored  with  the  bright 
shades  which  under  natural  conditions  indicate  freshness. 
/       Bleaching  with  chlorine  is  permanent,  the  dyestuff  being 
\  destroyed  by  the  chlorine  ;  but  bleaching  with  sulphurous  acid 
\  is  temporary,  because  the  milder  bleach  does  not  actually  de- 
stroy the  dyestuff,  but  merely  modifies  it,  and  in  time  the  natu- 
ral yellow  color  of  straw,  cotton,  and  linen  reappears.     The 
yellowing  of  straw  hats  during  the  summer  is  familiar  to  every 
one  ;  the  straw  is  merely  resuming  its  natural  color  which  had 
been  modified  by  the  sulphurous  acid  solution  applied  to  the 
straw  when  woven. 

222.  Why  the  Color  Returns.  Some  of  the  compounds 
formed  by  the  sulphurous  acid  bleaching  process  are  gradu- 
ally decomposed  by  sunlight,  and  in  consequence  the  original 
color  is  in  time  partially  restored.  The  portion  of  a  hat  pro- 


THE  REMOVAL   OF  STAINS  243 

tected  by  the  band  retains  its  fresh  appearance  because  the 
light  has  not  had  access  to  it.  Silks  and  other  fine  fabrics 
bleached  in  this  way  fade  with  age,  and  assume  an  unnatural 
color.  One  reason  for 'this  is  that  the  dye  used  to  color  the 
fabric  requires  a  clear  white  background,  and  loses  its  charac- 
teristic hues  when  its  foundation  is  yellow  instead  of  white. 
Then,  too,  dyestuffs  are  themselves  more  or  less  affected  by 
light,  and  fade  slowly  under  a  strong  illumination. 

Materials  which  are  not  exposed  directly  to  an  intense  and 
prolonged  illumination  retain  their  whiteness  for  a  long  time, 
and  hence  dress  materials  and  hats  which  have  been  bleached  / 
with  sulphurous  acid  should  be  protected  from  the  sun's  glare) 
when  not  in  use. 

223.  The  Removal  of  Stains.  Bleaching  powder  is  very 
useful  in  the  removal  of  stains  from  white  fabrics.  Ink  spots 
rubbed  with  lemon  juice  and  dipped  in  bleaching  solution 
fade  away  and  leave  on  the  cloth  no  trace  of  discoloration. 
Sometimes  these  stains  can  be  removed  by  soaking  in  milk,  v 
and  where  this  is  possible,  it  is  the  better  method. 

Bleaching  solution,  however,  while  valuable  in  the  removal 
of  some  stains,  is  unable  to  remove  paint  stains,  because  paints 
owe  their  color  to  mineral  matter,  and  on  this  chlorine  is 
powerless  to  act.  Paint  stains  are  best  removed  by  the  ap- 
plication of  gasoline  followed  by  soap  and  water. 


CHAPTER   XXIII 

DYEING 

224.  Dyes.     One  of  the  most  important  and  lucrative  in- 
dustrial processes  of  the  world  to-day  is  that  of  staining  and 
dyeing.      Whether  we  consider  the  innumerable  shades  of 
leather  used  in  shoes  and  harnesses  and  upholstery ;  the  mul- 
titude of  colors  in  the  paper  which  covers  our  walls  and  re- 
flects light  ranging  from  the  somber  to  the  gay,  and  from 
the   delicate   to  the  gorgeous ;  the   artificial   scenery  which 
adorns  the  stage  and  by  its  imitation  of  trees  and  flowers 
and  sky  translates  us  to  the  Forest  of  Arden ;  or  whether 
we  consider  the  uncounted  varieties   of  color  in  dress  ma- 
terials, in  carpets,  and  in  hangings,  we  are  dealing  with  sub- 
stances which  owe  their  beauty  to  dyes  and  dyestuffs. 

The  coloring  of  textile  fabrics,  such  as  cotton,  wool,  and 
silk,  far  outranks  in  amount  and  importance  that  of  leather, 
paper,  etc.,  and  hence  the  former  only  will  be  considered 
here ;  but  the  theories  and  facts  relative  to  textile  dyeing  are 
applicable  in  a  general  way  to  all  other  forms  as  well. 

225.  Plants  as  a  Source  of  Dyes.     Among  the  most  beauti- 
ful examples  of  man's  handiwork  are  the  baskets  and  blankets 
of  the  North  American  Indians,  woven  with  a  skill  which 
cannot   be   equaled  by   manufacturers,   and   dyed  in  mellow 
colors  with  a  few  simple  dyes  extracted  from   local  plants. 
The  magnificent  rugs  and  tapestries  of  Persia  and  Turkey, 
and  the  silks  of  India  and  Japan,  give  evidence  that  a  knowl- 
edge of  dyes  is  widespread  and  ancient.     Until  recently,  the 

244 


WOOL  AND   COTTON  DYEING  24$ 

vegetable  world  was  the  source  of  practically  all  coloring 
matter,  the  pulverized  root  of  the  madder  plant  yielding  the 
reds,  the  leaves  and  stems  of  the  indigo  plant  the  blues,  the 
heartwood  of  the  tropical  logwood  tree  the  blacks  and  grays, 
and  the  fruit  of  certain  palm  and  locust  trees  yielding  the 
soft  browns.  So  great  was  the  commercial  demand  for  dye- 
stuffs  that  large  areas  of  land  were  given  over  to  the  exclusive 
cultivation  of  the  more  important  dye  plants.  Vegetable  dyes 
are  now,  however,  rarely  used  because  about  the  year  1856 
it  was  discovered  that  dyes  could  be  obtained  from  coal  tar, 
the  thick  sticky  liquid  formed  as  a  by-product  in  the  manu- 
facture of  coal  gas.  These  artificial  coal-jtar,  or  aniline,  dyes 
have  practically  undisputed  sway  to-day,  and  the  vast  areas 
of  land  formerly  used  for  the  cultivation  of  vegetable  dyes 
are  now  free  for  other  purposes. 

226.  Wool  and  Cotton  Dyeing.  If  a  piece  of  wool  is  soaked 
in  a  solution  of  a  coal-tar  dye,  such  as  magenta,  the  fiber 
of  the  cloth  draws  some  of  the  dye  out  of  the  solution  and 
absorbs  it,  becoming  in  consequence  beautifully  colored.  The 
coloring  matter  becomes  "  part  and  parcel,"  as  it  were,  of 
the  wool  fiber,  because  repeated  washing  of  the  fabric  fails  to 
remove  the  newly  acquired  color ;  the  magenta  coloring  matter 
unites  chemically  with  the  fiber  of  the  wool,  and  forms  with 
it  a  compound  insoluble  in  water,  and  hence  fast  to  washing. 

But  if  cotton  is  used  instead  of  wool,  the  acquired  color  is 
very  faint,  and  washes  off  readily.  This  is  because  cotton 
fibers  possess  no  chemical  substance  capable  of  uniting  with 
the  coloring  matter  to  form  a  compound  insoluble  in  water. 

If  magenta  is  replaced  by  other  artificial  dyes, —  for  example, 
scarlets,  —  the  result  is  similar;  in  general,  wool  material  ab- 
sorbs dye  readily,  and  uniting  with  it  is  permanently  dyed. 
Cotton  material,  on  the  other  hand,  does  not  combine  chemically 
with  coloring  matter  and  therefore  is  only  faintly  tinged  with 


246  DYEING 

I  color,  and  loses  this  when  washed.  When  silk  and  linen  are 
tested,  it  is  found  that  the  former  behaves  in  a  general  way 
as  did  wool,  while  the  linen  has  more  similarity  to  the  cotton. 
That  vegetable  fibers,  such  as  cotton  and  linen,  should  act 
differently  toward  coloring  matter  from  animal  fibers,  such  as 
silk  and  wool,  is  not  surprising  when  we  consider  that  the 
chemical  nature  of  the  two  groups  is  very  different ;  vegetable 
fibers  contain  only  oxygen,  carbon,  and  hydrogen,  while  ani- 
mal fibers  always  contain  nitrogen  in  addition,  and  in  many 
cases  sulphur  as  well. 

227.  The  Selection  of  Dyes.  When  silk  and  wool,  cotton  and 
linen,  are  tested  in  various  dye  solutions,  it  is  found  that  the 
former  have,  in  general,  a  great  affinity  for  coloring  matter 
and  acquire  a  permanent  color,  but  that  cotton  and  linen,  on 
the  other  hand,  have  little  affinity  for  dyestuffs.  The  color 
acquired  by  vegetable  fibers  is,  therefore,  usually  faint. 

There  are,  of  course,  many  exceptions  to  the  general  state- 
ment that  animal  fibers  dye  readily  and  vegetable  fibers  poorly, 
because  certain  dyes  fail  utterly  with  woolen  and  silk  material 
and  yet  are  fairly  satisfactory  when  applied  to  cotton  and 
linen  fabrics.  Then,  too,  a  dye  which  will  color  silk  may  not 
have  any  effect  on  woo]  in  spite  of  the  fact  that  wool,  like 
silk,  is  an  animal  fiber ;  and  certain  dyestuffs  to  which  cotton 
responds  most  beautifully  are  absolutely  without  effect  on 
linen. 

The  nature  of  the  material  to  be  dyed  determines  the  color- 
ing matter  to  be  used;  in  dyeing  establishments  a  careful  ex- 
amination is  made  of  all  textiles  received  for  dyeing,  and  the 
particular  dyestuffs  are  then  applied  which  long  experience 
has  shown  to  be  best  suited  to  the  material  in  question. 
Where  "  mixed  goods,"  such  as  silk  and  wool,  or  cotton  and 
wool,  are  concerned,  the  problem  is  a  difficult  one,  and  the 
countless  varieties  of  gorgeously  colored  mixed  materials  give 


HOW   VARIETY  OF  COLOR  IS  SECURED  247 

evidence  of  high  perfection  in  the  art  of  dyeing  and  weav- 
ing. 

Housewives  who  wish  to  do  successful  home  dyeing  should 
therefore  not  purchase  dyes  indiscriminately,  but  should  select 
the  kind  best  suited  to  the  material,  because  the  coloring 
principle  which  will  remake  a  silk  waist  may  utterly  ruin  a 
woolen  skirt  or  a  linen  suit.  Powders  designed  for  special 
purposes  may  be  purchased  from  druggists. 

228.  Indirect  Dyeing.     We  have  seen  that  it  is  practically 
impossible  to  color  cotton  and  linen  in  a  simple  manner  with 
any  degree  of  permanency,  because  of  the  lack  of  chemical 
action  between  vegetable  fibers  and   coloring  matter.     But 
the  varied  uses  to  which  dyed  articles  are  put  make  fastness 
of  color  absolutely  necessary.     A  shirt,  for  example,  must  not 
be  discolored  by  perspiration,  nor  a  waist  faded  by  washing, 
nor  a  carpet  dulled  by  sweeping  with  a  dampened  broom. 
In  order  to  insure  permanency  of  dyes,  an  indirect  method  was 
originated  which  consisted  of  adding  to  the  fibers  a  chemical 
capable  of  acting  upon  the  dye  and  forming  with  it  a  colored 
compound  insoluble  in  water,  and  hence  '"safe."      For  ex- 
ample, cotton  material  dyed  directly  in  logwood  solution  has 
almost  no  value,  but  if  it  is  soaked  in  a  solution  of  oxalic  acid 
and  alum  until  it  becomes  saturated  with  the  chemicals,  and 
is  then  transferred  to  a  logwood  bath,  the  color  acquired  is 
fast  and  beautiful. 

This  method  of  indirect  dyeing  is  known  as  the  mordanting 
process ;  it  consists  of  saturating  the  fabric  to  be  dyed  with 
chemicals  which  will  unite  with  -the  coloring  matter  to  form 
compounds  unaffected  by  water.  The  chemicals  are  called 
mordants. 

229.  How  Variety  of  Color  is  Secured.     The  color  which  is 
fixed  on  the  fabric  as  a  result  of  chemical  action  between  mor- 
dant and  dye  is  frequently  very  different  from  that  of  the  dye 


248  DYEING 

itself.  Logwood  dye  when  used  alone  produces  a  reddish 
brown  color  of  no  value  either  for  beauty  or  permanence; 
but  if  the  fabric  to  be  dyed  is  first  mordanted  with  a  solution 
of  alum  and  oxalic  acid  and  is  then  immersed  in  a  logwood 
bath,  it  acquires  a  beautiful  blue  color. 

Moreover,  since  the  color  acquired  depends  upon  the  mor- 
dant as  well  as  upon  the  dye,  it  is  often  possible  to  obtain  a 
wide  range  of  colors  by  varying  the  mordant  used,  the  dye 
remaining  the  same.  For  example,  with  alum  and  oxalic  acid 
as  a  mordant  and  logwood  as  a  dye,  blue  is  obtained;  but 
with  a  mordant  of  ferric  sulphate  and  a  dye  of  logwood, 
blacks  and  grays  result.  Fabrics  immersed  directly  in  aliz- 
arin acquire  a  reddish  yellow  tint ;  when,  however,  they  are 
mordanted  with  certain  aluminium  compounds  they  acquire 
a  brilliant  Turkey  red,  when  mordanted  with  chromium  com- 
pounds, a  maroon,  and  when  mordanted  with  iron  compounds, 
the  various  shades  of  purple,  lilac,  and  violet  result. 

230.  Color  Designs  in  Cloth.  It  is  thought  that  the  earliest 
attempts  at  making  "  fancy  materials  "  consisted  in  painting 
designs  on  a  fabric  by  means  of  a  brush.  In  more  recent 
times  the  design  was  cut  in  relief  on  hard  wood,  the  relief 
being  then  daubed  with  coloring  matter  and  applied  by  hand 
to  successive  portions  of  the  cloth.  The  most  modern  method 
of  design-making  is  that  of  machine  or  roller  printing.  In 
this,  the  relief  blocks  are  replaced  by  engraved  copper  rolls 
which  rotate  continuously  and  in  the  course  of  their  rotation 
automatically  receive  coloring  matter  on  the  engraved  por- 
tion. The  cloth  to  be  printed  is  then  drawn  uniformly  over 
the  rotating  roll,  receiving  color  from  the  engraved  design ; 
in  this  way,  the  color  pattern  is  automatically  printed  on 
the  cloth  with  perfect  regularity.  In  cases  where  the  fabrics 
do  not  unite  directly  with  the  coloring  matter,  the  design  is 
supplied  with  a  mordant  and  the  impression  made  on  the 


COLOR  DESIGNS  IN  CLOTH  249 

fabric  is  that  of  the  mordant ;  when  the  fabric  is  later  trans- 
ferred to  a  dye  bath,  the  mordanted  portions,  represented  by 
the  design,  unite  with  the  coloring  matter  and  thus  form  the 
desired  color  patterns. 

Unless  the  printing  is  well  done,  the  coloring  matter  does 
not  thoroughly  penetrate  the  material,  and  only  a  faint  blurred 
design  appears  on  the  back  of  the  cloth ;  the  gaudy  designs 
of  cheap  calicoes  and  ginghams  often  do  not  show  at  all  on 
the  under  side.  Such  carelessly  made  prints  are  not  fast  to 
washing  or  light,  and  soon  fade.  But  in  the  better  grades  of 
material  the  printing  is  well  done,  and  the  color  designs  are 
fairly  fast,  and  a  little  care  in  the  laundry  suffices  to  eliminate 
any  danger  of  fading. 

Color  designs  of  the  greatest  durability  are  produced  by 
the  weaving  together  of  colored  yarns.  When  yarn  is  dyed, 
the  coloring  matter  penetrates  to  every  part  of  the  fiber,  and 
hence  the  patterns  formed  by  the  weaving  together  of  well- 
dyed  yarns  are  very  fast  to  light  and  water. 

If  the  color  designs  to  be  woven  in  the  cloth  are  intricate, 
complex  machinery  is  necessary  and  skillful  handwork ; 
hence,  patterns  formed  by  the  weaving  of  colored  yarns  are 
expensive  and  less  common  than  printed  fabrics. 


CHAPTER   XXIV 

CHEMICALS   AS    DISINFECTANTS   AND   PRESERVATIVES 

231.  The  prevention  of  disease  epidemics  is  one  of  the  most 
striking  achievements  of  modern  science.  Food,  clothing, 
furniture,  and'  other  objects  contaminated  in  any  way  by 
disease  germs  may  be  disinfected  by  chemicals  or  by  heat, 
and  widespread  infection  from  persons  suffering  with  a  con- 
tagious disease  may  be  prevented. 

When  disease  germs  are  within  the  body,  the  problem  is 
far  from  simple,  because  chemicals  which  would  effectively 
destroy  the  germs  would  be  fatal  to  life  itself.  But  when 

germs  are  outside  the 
body,  as  in  water  or 
milk,  or  on  clothing, 
dishes,  or  furniture, 
they  can  be  easily 
killed.  One  of  the 
best  methods  of  de- 
stroying germs  is  to 
subject  them  to  in- 
tense heat.  Contam- 

FlG.  160.  — Pasteurizing  apparatus,  an  arrangement  mated  Water  IS  made 
by  which  milk  is  conveniently  heated  to  destroy  safe  by  boiling  f or  a 
disease  germs. 

few    minutes,    because 

the  strong  heat  destroys  the  disease-producing  germs. 
Scalded  or  Pasteurized  milk  saves  the  lives  of  scores  of 
babies,  because  the  germs  of  summer  complaint  which  lurk 

250 


PERSONAL   DISINFECTION-  251 

in  poor  milk  arc  killed  and  rendered  harmless  in  the  process 
of  scalding.  Dishes  used  by  consumptives,  and  persons  suf- 
fering from  contagious  diseases,  can  be  made  harmless  by 
thorough  washing  in  thick  suds  of  almost  boiling  water. 

The  bedding  and  clothing  of  persons  suffering  with  diph- 
theria, tuberculosis,  and  other  germ  diseases  should  always  be 
boiled  and  hung  to  dry  in  the  bright  sunlight.  Heat  and  sun- 
shine are  two  of  the  best  disinfectants. 

232.  Chemicals.     Objects,  such  as  furniture,  which  cannot 
be  boiled,  are  disinfected  by  the  use  of  any  one  of  several 
chemicals,  such  as  sulphur,  carbolic  acid,  chloride  of  lime, 
corrosive  sublimate,  etc. 

One  of  the  simplest  methods  of  disinfecting  consists  in 
burning  sulphur  in  a  room  whose  doors,  windows,  and  key- 
holes have  been  closed,  so  that  the  burning  fumes  cannot 
escape,  but  remain  in  the  room  long  enough  to  destroy  disease 
germs.  This  is  probably  the  most  common  means  of  fumi- 
gation. 

For  general  purposes,  carbolic  acid  is  one  of  the  very  best 
disinfectants,  but  must  be  used  with  caution,  as  it  is  a  deadly 
poison  except  when  very  dilute. 

Chloride  of  lime  when  exposed  to  the  air  and  moisture  slowly 
gives  off  chlorine,  and  can  be  used  as  a  disinfectant  because  the 
gas  thus  set  free  attacks  germs  and  destroys  them.  For  this 
reason  chloride  of  lime  is  an  excellent  disinfectant  of  drain- 
pipes. Certain  bowel  troubles,  such  as  diarrhoea,  are  due  to 
microbes,  and  if  the  waste  matter  of  a  person  suffering  from 
this  or  similar  diseases  is  allowed  passage  through  the  drain- 
age system,  much  damage  may  be  done.  But  a  small  amount 
of  chloride  of  lime  in  the  closet  bowl  will  insure  disinfection. 

233.  Personal  Disinfection.     The  hands  may  gather  germs 
from  any  substances  or  objects  with  which  they  come  in  con- 
tact; hence  the  hands  should  be  washed  with  soap  and  water, 


252     CHEMICAL  DISINFECTANTS  AND  PRESERVATIVES 

and  especially  before  eating.  Physicians  who  perform  opera- 
tions wash  not  only  their  hands,  but  their  instruments,  steril- 
izing the  latter  by  placing  them  in  boiling  water  for  several 
minutes. 

Cuts  and  wounds  allow  easy  access  to  the  body ;  a  small 
cut  has  been  known  to  cause  death  'because  of  the  bacteria 
which  found  their  way  into  the  open  wound  and  produced 
disease.  In  order  to  destroy  any  germs  which  may  have 
entered  into  the  cut  from  the  instrument,  it  is  well  to  wash  out 
the  wound  with  some  mild  disinfectant,  such  as  very  dilute 
carbolic  acid  or  hydrogen  peroxide,  and  then  to  bind  the 
wound  with  a  clean  cloth,  to  prevent  later  entrance  of  germs. 

234.  Chemicals  as  Food  Preservatives.  The  spoiling  of 
meats  and  soups,  and  the  souring  of  milk  and  preserves,  are 
due  to  germs  which,  like  those  producing  disease,  can  be 
destroyed  by  heat  and  by  chemicals. 

Milk  heated  to  the  boiling  point  does  not  sour  readily,  and 
successful  canning  consists  in  cooking  fruits  and  vegetables 
until  all  the  germs  are  killed,  and  then  sealing  the  cans  so 
that  germs  from  outside  cannot  find  entrance  and  undo  the 
work  of  the  canner. 

Some  dealers  and  manufacturers  have  learned  that  certain 
chemicals  will  act  as  food  preservatives,  and  hence  they  have 
replaced  the  safe  method  of  careful  canning  by  the  quicker 
and  simpler  plan  of  adding  chemicals  to  food.  Catchup, 
sauces,  and  jellies  are  now  frequently  preserved  in  this  way. 
But  the  chemicals  which  destroy  bacteria  frequently  injure 
the  consumer  as  well.  And  so  much  harm  has  been  done 
by  food  preservatives  that  the  pure  food  laws  require  that 
cans  and  bottles  contain  a  labeled  statement  of  the  kind 
and  quantity  of  chemicals  used. 

Even  milk  is  not  exempt,  but  is  doctored  to  prevent  sour- 
ing, the  preservative  most  generally  used  by  milk  dealers 


THE  PRESERVATION  OF   WOOD  AND  METAL      253 

being  formaldehyde.  The  vast  quantity  of  milk  consumed  by 
young  and  old,  sick  and  well,  makes  the  use  of  formaldehyde 
a  serious  menace  to  health,  because  no  constitution  can  endure 
the  injury  done  by  the  constant  use  of  preservatives. 

The  most  popular  and  widely  used  preservatives  of  meats 
are  borax  and  boric  acid.  These  chemicals  not  only  arrest 
decay,  but  partially  restore  to  old  and  bad  meat  the  appear- 
ance of  freshness ;  in  this  way  unscrupulous  dealers  are  able 
to  sell  to  the  public  in  one  form  or  other  meats  which  may 
have  undergone  partial  decomposition ;  sausage  frequently 
contains  partially  decomposed  meat,  restored  as  it  were  by 
chemicals. 

In  jams  and  catchups  there  is*  abundant  opportunity  for 
preservatives ;  badly  or  partially  decayed  fruits  are  sometimes 
disinfected  and  used  as  the  basis  of  foods  sold  by  so-called 
good  dealers.  Benzoate  of  soda,  and  salicylic  acid  are  the 
chemicals  most  widely  employed  for  this  purpose,  with  coal- 
tar  dyes  to  simulate  the  natural  color  of  the  fruit. 

Many  of  the  cheap  candies  sold  by  street  venders  are  not  fit 
for  consumption,  since  they  are  not  only  made  of  bad  material, 
but  are  frequently  in  addition  given  a  light  dipping  in  varnish 
as  a  protection  against  the  decaying  influences  of  the  atmos- 
phere. 

The  only  wise  preservatives  are  those  long  known  and 
employed  by  our  ancestors ;  salt,  vinegar,  and  spices  are  all 
food  preservatives,  but  they  are  at  the  same  time  substances 
which  in  small  amounts  are  not  injurious  to  the  body.  Smoked 
herring  and  salted  mackerel  are  chemically  preserved  foods, 
but  they  are  none  the  less  safe  and  digestible. 

235.  The  Preservation  of  Wood  and  Metal.  The  decaying 
of  wood  and  the  rusting  of  metal  are  due  to  the  action  of  air 
and  moisture.  When  wood  and  metal  are  surrounded  with  a 
covering  which  neither  air  nor  moisture  can  penetrate,  decay 


254     CHEMICAL  DISINFECTANTS  AND  PRESERVATIVES 

and  rust  are  prevented.  Paint  affords  such  a  protective 
covering.  The  main  constituent  of  paint  is  a  compound  of 
white  lead  or  other  nretallic  substance;  this  is  mixed  with 
linseed  oil  or  its  equivalent  in  order  that  it  may  be  spread 
over  wood  and  metal  in  a  thin,  even  coating.  After  the 
mixture  has  been  applied,  it  hardens  and  forms  a 'tough  skin 
fairly  impervious  to  weathering.  For  the  sake  of  ornamenta- 
tion, various  colored  pigments  are  added  to  the  paint  and 
give  variety  of  effect. 

Railroad  ties  and  street  paving  blocks  are  ordinarily  pro- 
tected by  oil  rather  than  paint.  Wood  is  soaked  in  creosote 
oil  until  it  becomes  thoroughly  saturated  with  the  oily 
substance.  The  pores  of  the  wood  are  thus  closed  to  the 
entrance  of  air  and  moisture,  and  decay  is  avoided.  Wood 
treated  in  this  way  is  very  durable.  Creosote  is  poisonous  to 
insects  and  many  small  animals,  and  thus  acts  as  a  preserva- 
tion not  only  against  the  elements  but  against  animal  life  as 
well. 


CHAPTER   XXV 

DRUGS  AND  PATENT  MEDICINES 

236.  Stimulants  and  Narcotics.  Man  has  learned  not  only 
the  action  of  substances  upon  each  other,  such  as  bleaching 
solution  upon  coloring  matter,  washing  soda  upon  grease, 
acids  upon  bases,  but  also  the  effect  which  certain  chemicals 
have  upon  the  human  body. 

Drugs  and  their  varying  effects  upon  the  human  system 
have  been  known  to  mankind  from  remote  ages ;  in  the  early 
days,  familiar  leaves,  roots,  and  twigs  were  steeped  in  water 
to  form  medicines  which  served  for  the  treatment  of  all  ail- 
ments. In  more  recent  times,  however,  these  simple  herb 
teas  have  been  supplanted  by  complex  drugs,  and  now  medi- 
cines are  compounded  not  only  from  innumerable  plant 
products,  but  from  animal  and  mineral  matter  as  well.  Qui- 
nine, rhubarb,  and  arnica  are  examples  of  purely  vegetable 
products ;  iron,  mercury,  and  arsenic  are  equally  well  known 
as  distinctly  mineral  products,  while  cod-liver  oil  is  the  most 
familiar  illustration  of  an  animal  remedy.  Ordinarily  a  com- 
bination of  products  best  serves  the  ends  of  the  physician. 

Substances  which,  like  cod-liver  oil,  serve  as  food  to  a 
worn-out  body,  or,  like  iron,  tend  to  enrich  the  blood,  or,  like 
quinine,  aid  in  bringing  an  abnormal  system  to  a  healthy  con- 
dition, are  valuable  servants  and  cannot  be  entirely  dispensed 
with  so  long  as  man  is  subject  to  disease. 

But  substances  which,  like  opium,  laudanum,  and  alcohol, 
are  not  required  by  the  body  as  food,  or  as  a  systematic, 
intelligent  aid  to  recovery,  but  are  taken  solely  for  the  stimu- 
lus aroused  or  for  the  insensibility  induced,  arc  harmful  to 

255 


256  DRUGS  AND   PATENT  MEDICINES 

man,  and  cannot  be  indulged  in  by  him  without  ultimate 
mental,  moral,  and  physical  loss.  Substances  of  the  latter 
class  are  known  as  narcotics  and  stimulants. 

237.  The  Cost  of  Health.     In  the  physical  as  in  the  finan- 
cial world,  nothing  is  to  be  had  without  a  price.     Vigor, 
endurance,  and  mental  alertness  are  bought  by  hygienic  liv- 
ing ;  that  is,  by  proper  food,  fresh  air,  exercise,  cleanliness, 
and  reasonable  hours.     Some  people  wish  vigor,  endurance, 
etc.,  but  are  unwilling  to  live  the  life  which  will  develop  these 
qualities.     Plenty  of  sleep,  exercise,  and  simple  food  all  tend 
to  lay  the  foundations  of  health.     Many,  however,  are  not 
willing  to  take  the  care  necessary  for  healthful  living,  because 
it  would  force  them  to  sacrifice  some  of  the  hours  of  pleasure. 
Sooner  or  later,  these  pleasure-seekers  begin  to  feel  tired  and 
worn,  and  some  of  them  turn  to  drugs  and  narcotics  for  arti- 
ficial strength.     At  first  the  drugs  seem  to  restore  the  lost 
energy,  and  without  harm ;  however,  the  cost  soon  proves  to 
be  one  of  the  highest  Nature  ever  demands. 

238.  The  Uncounted  Cost.      The   first  and   most   obvious 
effect  of  opium,  for  example,  is  to  deaden  pain  and  to  arouse 
pleasure ;    but  while  the  drug  is  producing   these   soothing 
sensations,  it  interferes  with  bodily  functions.     Secretion,  di- 
gestion, absorption  of  food,  and  the  removal  of  waste  matters 
are  hindered.      Continued  use  of  the  drug  leads  to  headache, 
exhaustion,  nervous  depression,  and  heart  weakness.     There 
is  thus  a  heavy  toll  reckoned  against  the  user,  and  the  cred- 
itor is  relentless  in  demanding  payment. 

Moreover,  the  respite  allowed  by  a  narcotic  is  exceedingly 
brief,  and  a  depression  which  is  long  and  deep  inevitably 
follows.  In  order  to  overcome  this  depression,  recourse  is 
usually  had  to  a  further  dose,  and  as  time  goes  on,  the  inter- 
vals of  depression  become  more  frequent  and  lasting,  and 
the  necessity  to  overcome  them  increases.  Thus  without  in- 


PATENT  MEDICINES,   COUGH  SIRUPS  257 

tention  one  finds  one's  self  bound  to  the  drug,  its  fast  victim. 
The  sanatoria  of  our  country  are  crowded  with  people  who 
are  trying  to  free  themselves  of  a  drug  habit  into  which  they 
have  drifted  unintentionally  if  not  altogether  unknowingly. 
What  is  true  of  opium  is  equally  applicable  to  other  narcotics. 

239.  The   Right  Use  of  Narcotics.     In  the  hands  of  the 
physician,  narcotics  are  a  great  blessing.     In  some  cases,  by 
relieving  pain,  they  give  the  system  the  rest  necessary  for 
overcoming  the  cause  of  the  pain.     Only  those  who  know  of 
the  suffering  endured  in  former  times  can  fully  appreciate  the 
decrease  in  pain  brought  about  by  the  proper  use  of  narcotics. 

240.  Patent  Medicines,  Cough  Sirups.    A  reputable  physician 
is  solicitous  regarding  the  permanent  welfare  of  his  patient 
and  administers  carefully  chosen  and  harmless  drugs.     Mere 
medicine  venders,   however,   ignore  the    good   of    mankind, 
and  flood  the  market  with  cheap  patent  preparations  which 
delude  and  injure  those  who  purchase,  but  bring  millions  of 
dollars  to  those  who  manufacture. 

Practically  all  of  these  patent,  or  proprietary,  preparations 
contain  a  large  proportion  of  narcotics  or  stimulants,  and  hence 
the  benefit  which  they  seem  to  afford  the  user  is  by  no  means 
genuine ;  examination  shows  that  the  relief  brought  by  them  is 
due  either  to  a  temporary  deadening  of  sensibilities  by  narcotics 
or  to  a  fleeting  stimulation  by  alcohol  and  kindred  substances. 

Among  the  most  common  ailments  of  both  young  and  old 
are  coughs  and  colds;  hence  many  patent  cough  mixtures 
have  been  manufactured  and  placed  on  the  market  for  the 
consumption  of  a  credulous  public.  Such  "quick  cures" 
almost  invariably  contain  one  or  more  narcotic  drugs,  and 
not  only  do  not  relieve  the  cold  permanently,  but  occasion 
subsequent  disorders.  Even  lozenges  and  pastilles  are  not 
free  from  fraud,  but  have  a  goodly  proportion  of  narcotics, 
containing  in  some  cases  chloroform,  morphine,  and  ether. 
CL.  GEN.  sci.  —  1 7 


258  DRUGS  AND  PATENT  MEDICINES 

The  widespread  use  of  patent  cough  medicines  is  due 
largely  to  the  fact  that  many  persons  avoid  consulting  a 
physician  about  so  trivial  an  ailment  as  an  ordinary  cold,  or 
are  reluctant  to  pay  a  medical  fee  for  what  seems  a  slight 
indisposition  and  hence  attempt  to  doctor  themselves. 

Catarrh  is  a  very  prevalent  disease  in  America,  and  conse- 
quently numerous  catarrh  remedies  have  been  devised,  most 
of  which  contain  in  a  disguised  form  the  pernicious  drug, 
cocaine.  Laws  have  been  enacted  which  require  on  the 
labels  a  declaration  of  the  contents  of  the  preparation,  both 
as  to  the  kind  of  drug  used  and  the  amount,  and  the  choice 
of  accepting  or  refusing  such  mixtures  is  left  to  the  indi- 
vidual. But  the  great  mass  of  people  are  ignorant  of  the 
harmful  nature  of  drugs  in  general,  and  hence  do  not  even 
read  the  self-accusing  label,  or  if  they  do  glance  at  it,  fail  to 
comprehend  the  dangerous  nature  of  the  drugs  specified  there. 
In  order  to  safeguard  the  uninformed  purchaser  and  to  re- 
strict the  manufacture  of  harmful  patent  remedies,  some  states 
limit  the  sale  of  all  preparations  containing  narcotics  and  thus 
give  free  rein  to  neither  consumer  nor  producer. 

241.  Soothing  Sirups;  Soft  Drinks.  The  development  of 
a  race  is  limited  by  the  mental  and  physical  growth  of  its 
children,  and  yet  thousands  of  its  children  are  annually  stunted 
and  weakened  by  drugs,  because  most  colic  cures,  teething  con- 
coctions, and  soothing  syrups  are  merely  agreeably  flavored 
drug  mixtures.  Those  who  have  used  such  preparations 
freely,  know  that  a  child  usually  becomes  fretful  and  irritable 
between  doses,  and  can  be  quieted  only  by  larger  and  more 
frequent  supplies.  A  habit  formed  in  this  way  is  difficult  to 
overcome,  and  many  a  child  when  scarcely  over  its  babyhood 
has  a  craving  which  in  later  years  may  lead  to  systematic 
drug  taking.  And  even  though  the  pernicious  drug  craving 
is  not  created,  considerable  harm  is  done  to  the  child,  because 


HEADACHE  POWDERS  259 

its  body  is  left  weak  and  non-resistant  to  diseases  of  infancy 
and  childhood. 

Many  of  our  soft  drinks  contain  narcotics.  The  use  of  the 
coca  leaf  and  the  kola  nut  for  such  preparations  has  increased 
very  greatly  within  the  last  few  years,  and  doubtless  legisla- 
tion will  soon  be  instituted  against  the  indiscriminate  sale  of 
soft  drinks. 

242.  Headache  Powders.  The  stress  and  strain  of  modern 
life  has  opened  wide  the  door  to  a  multitude  of  bodily  ills, 
among  which  may  be  mentioned  headache.  Work  must  be 
done  and  business  attended  to,  and  the  average  sufferer  does 
not  take  time  from  his  vocation  to  investigate  the  cause  of 
the  headache,  but  unthinkingly  grasps  at  any  remedy  which 
will  remove  the  immediate  pain,  and  utterly  disregards  later 
injury.  The  relief  afforded  by  most  headache  mixtures  is 
due  to  the  presence  of  antipyrin  or  acetanilid,  and  it  has 
been  shown  conclusively  that  these  drugs  weaken  heart 
action,  diminish  circulation,  reduce  the  number  of  red  cor- 
puscles in  the  blood,  and  bring  on  a  condition  of  chronic 
anemia.  Pallid  cheeks  and  blue  lips  are  visible  evidence  of 
the  too  frequent  use  of  headache  powders. 

The  labels  required  by  'law  are  often  deceptive  and  convey 
no  adequate  idea  of  the  amount  of  drug  consumed ;  for  ex- 
ample, 240  grains  of  acetanilid  to  an  ounce  seems  a  small 
quantity  of  drug  for  a  powder,  but  when  one  considers  that 
there  are  only  480  grains  in  an  ounce,  it  will  be  seen  that 
each  powder  is  one  half  acetanilid. 

Powders  taken  in  small  quantities  and  at  rare  intervals 
are  apparently  harmless ;  but  they  never  remove  the  cause 
of  the  trouble,  and  hence  the  discomfort  soon  returns  with  re- 
newed force.  Ordinarily,  hygienic  living  will  eliminate  the 
source  of  the  trouble,  but  if  it  does  not,  a  physician  should 
be  consulted  and  medicine  should  be  procured  from  him 


2(50  DRUGS  AND  PATENT  MEDICINES 

which  will  restore  the  deranged  system  to  its  normal  healthy 
condition. 

243.    Other  Deceptions.     Nearly  all  patent  medicines  con- 
tain some  alcohol,  and  in  many,  the  quantity  of  alcohol  is  far 


FlG.  161.  —  Diagram  showing  the  amount  of  alcohol  in  some  alcoholic  drinks  and  in 
one  much  used  patent  medicine. 

in  excess  of  that  found  in  the  strongest  wines.  Tonics  and 
bitters  advertised  as  a  cure  for  spring  fever  and  a  worn-out 
system  are  scarcely  more  than  cheap  cocktails,  as  one  writer 
has  derisively  called  them,  and  the  amount  of  alcohol  in  some 
widely  advertised  patent  remedies  is  alarmingly  large  and 
almost  equal  to  that  of  strong  whisky. 

Some  conscientious  persons  who  would  not  touch  beer, 
wine,  whisky,  or  any  other  intoxicating  drink  consume  patent 
remedies  containing  large  quantities  of  alcohol  and  thus 
unintentionally  expose  themselves  to  mental  and  physical 
danger.  In  all  cases  of  bodily  disorder,  the  only  safe  course 
is  to  consult  a  physician  who  has  devoted  himself  to  the  study 
of  the  body  and  the  methods  by  which  a  disordered  system 
may  be  restored  to  health. 


CHAPTER   XXVI 

NITROGEN   AND   ITS   RELATION   TO   PLANTS 

244.  Nitrogen.     A  substance   which    plays   an    important 
part  in  animal  and  plant  life  is  nitrogen.     Soil  and  the  ferti- 
lizers which  enrich  it,  the  plants  which  grow  on  it,  and  the 
animals  which  feed  on  these,  all  contain  nitrogen  or  nitrog- 
enous compounds.      The    atmosphere,  which    we  ordinarily 
think  of  as  a  storehouse  of  oxygen,  contains  far  more  nitrogen 
than  oxygen,  since  four  fifths  of  its  whole  weight  is  made  up 
of  this  element. 

Nitrogen  is  colorless,  odorless,  and  tasteless.  Air  is  com- 
posed chiefly  of  oxygen  and  nitrogen ;  if,  therefore,  the  oxy- 
gen in  a  vessel  filled  with  air  can  be  made  to  unite  with  some 
other  substance  or  can  be  removed,  there  will  be  a  residue  of 
nitrogen.  This  can  be  done  by  floating  on  water  a  light  dish 
containing  phosphorus,  then  igniting  the  phosphorus,  and 
placing  an  inverted  jar  over  the  burning  substance.  The 
phosphorus  in  burning  unites  with  the  oxygen  of  the  air  and 
hence  the  gas  that  remains  in  the  jar  is  chiefly  nitrogen.  It 
has  the  characteristics  mentioned  above  and,  in  addition,  does 
not  combine  readily  with  other  substances. 

245.  Plant  Food.     Food,  is  the  source  of  energy  in  every 
living  thing  and  is  essential  to  both  animal  and   plant  life. 
Plants   get  their   food  from  the  lifeless  matter  which  exists 
in   the  air  and   in  the   soil ;    while  animals  get    their  food 
from  plants.     It  is  true  that  man  and  many  other  animals  eat 

261 


262        NITROGEN  AND  ITS  RELATION  TO  PLANTS 

fleshy  foods  and  depend  upon  them  for  partial  sustenance,  but 
the  ultimate  source  of  all  animal  food  is  plant  life,  since  meat- 
producing  animals  live  upon  plant  growth. 

Plants  get  their  food  from  the  air,  the  soil,  and  moisture. 
From  the  air,  the  leaves  take  carbon  dioxide  and  water  and 
transform  them  into  starchy  food  ;  from  the  soil,  the  roots  take 
water  rich  in  mineral  matters  dissolved  from  the  soil.  From 
the  substances  thus  gathered,  the  plant  lives  and  builds  up 
its  structure. 

A  food  substance  necessary  to  plant  life  and  growth  is  ni- 
trogen.    Since  a  vast  store  of  nitrogen  exists  in  the  air,  it 
would  seem  that  plants  should  never  lack  for  this  food,  but 
lost  plants  are  unable  to  make  use  of  the  boundless  store  of 
Atmospheric  nitrogen,  because  they  do  not  possess  the  power 
>f  abstracting  nitrogen  from  the  air.     For  this  reason,  they 
tave  to  depend  solely  upon   nitrogenous  compounds  which 
/are  present  in  the  soil  and  are  soluble  in  water.     The  soluble 
nitrogenous  soil  compounds  are  absorbed  by  roots  and  are 
utilized  by  plants  for  food. 

246.  The  Poverty  of  the  Soil.  Plant  roots  are  constantly 
taking  nitrogen  and  its  compounds  from  the  soil.  If  crops 
which  grow  from  the  soil  are  removed  year  after  year,  the  soil 
becomes  poorer  in  nitrogen,  and  finally  possesses  too  little  of  it 
to  support  vigorous  and  healthy  plant  life.  The  nitrogen  of 
the  soil  can  be  restored  if  we  add  to  it  a  fertilizer  containing 
nitrogen  compounds  which  are  soluble  in  water.  Decayed 
vegetable  matter  contains  large  quantities  of  nitrogen  com- 
pounds, and  hence  if  decayed  vegetation  is  placed  upon  soil 
or  is  plowed  into  soil,  it  acts  as  a  fertilizer,  returning  to  the 
soil  what  was  taken  from  it.  Since  man  and  all  other  animals 
subsist  upon  plants,  their  bodies  likewise  contain  nitrogenous 
substances,  and  hence  manure  and  waste  animal  matter  is 
valuable  as  a  fertilizer  or  soil  restorer. 


ARTIFICIAL  FERTILIZERS 


263 


247.  Bacteria  as  Nitrogen  Gatherers.  Soil  from  which  crops 
are  removed  year  after  year  usually  becomes  less  fertile,  but 
the  soil  from  which  crops 
of  clover,  peas,  beans,  or 
alfalfa  have  been  removed 
are  richer  in  nitrogen  rather 
than  poorer.  This  is  because 
the  roots  of  these  plants 
often  have  on  them  tiny 
swellings,  or  tubercles,  in 
which  millions  of  certain 
bacteria  live  and  multiply. 


FIG.  162.  —  Roots  of  soy  bean  having  tuber- 
cle-bearing bacteria. 


These  bacteria  have  the  re- 
markable power  of  taking 
free  nitrogen  from  the  air  in 

the  soil  and  of  combining  it  with  other  substances  to  form  com- 
pounds which  plants  can  use.  The  bacteria-made  compounds 
dissolve  in  the  soil  water  and  are  absorbed  into  the  plant  by  the 
roots.  So  much  nitrogen-containing  material  is  made  by  the 
root  bacteria  of  plants  of  the  pea  family  that  the  soil  in  which 
they  grow  becomes  somewhat  richer  in  nitrogen,  and  if  plants 
which  cannot  make  nitrogen  are  subsequently  planted  in  such 
a  soil,  they  find  there  a  store  of  nitrogen.  A  crop  of  peas, 
beans,  or  clover  is  equivalent  to  nitrogenous  fertilizer  and 
helps  to  make  ready  the  soil  for  other  crops. 

248.  Artificial  Fertilizers.  Plants  need  other  foods  besides 
nitrogen,  and  they  exhaust  the  soil  not  only  of  nitrogen,  but 
also  of  phosphorus  and  potash,  since  large  quantities  of  these 
are  necessary  for  plant  life.  There  are  many  other  sub- 
stances absorbed  from  the  soil  by  the  plant,  namely,  iron, 
sodium,  calcium,  magnesium,  but  these  are  used  in  smaller 
quantities  and  the  supply  in  the  soil  does  not  readily  become 
exhausted, 


264       NITROGEN  AND  ITS  RELATION  TO  PLANTS 

Commercial  fertilizers  generally  contain  nitrogen,  phospho- 
rus, and  potash  in  amounts  varying  with  the  requirements  of 
the  soil.  Wheat  requires  a  large  amount  of  phosphorus  and 
quickly  exhausts  the  ground  of  that  food 
stuff;  a  field  which  has  supported  a  crop 
of  wheat  is  particularly  poor  in  phosphorus, 
and  a  satisfactory  fertilizer  for  that  land 
would  necessarily  contain  a  large  percent- 
age of  phosphorus.  The  fertilizer  to  be 
used  in  a  soil  depends  upon  the  character 
of  the  soil  and  upon  the  crops  previously 
grown  on  it. 

The  quantity  of  fertilizer  needed  by  the 
farmers  of  the  world  is  enormous,  and 
the  problem  of  securing  the  necessary  sub- 
stances in  quantities  sufficient  to  satisfy 
the  demand  bids  fair  to  be  serious.  But 
modern  chemistry  is  at  work  on  the  prob- 
lem, and  already  it  is  possible  to  make 
some  nitrogen  compounds  on  a  commercial 
scale.  When  nitrogen  gas  is  in  contact 
with  heated  calcium  carbide,  a  reaction  takes  place  which 
results  in  the  formation  of  calcium  nitride,  a  compound  suit- 
able for  enriching  the  soil.  There  are  other  commercial 
methods  for  obtaining  nitrogen  compounds  which  are  suit- 
able for  absorption  by  plant  roots. 

Phosphorus  is  obtained  from  bone  ash  and  from  phosphate 
rock  which  is  widely  distributed  over  the  surface  of  the  earth. 
Bone  ash  and  thousands  of  tons  of  phosphate  rock  are  treated 
with  sulphuric  acid  to  form  a  phosphorus  compound  which  is 
soluble  in  soil  water  and  which,  when  added  to  soil,  will  be 
usable  by  the  plants  growing  there. 

The  other  important  ingredient  of  most  fertilizers  is  potash. 


FIG.  163.  —  Water  cul- 
tures of  buckwheat : 
I,  with  all  the  food 
elements ;  2,  without 
potash ;  3,  without 
nitrates. 


ARTIFICIAL  FERTILIZERS  26$ 

Wood  ashes  are  rich  in  potash  and  are  a  valuable  addition  to 
the  soil.  But  the  amount  of  potash  thus  obtained  is  far  too 
limited  to  supply  the  needs  of  agriculture  ;  and  to-day  the 
main  sources  of  potash  are  the  vast  deposits  of  potassium 
salts  found  in  Prussia. 

Although  Germany  now  furnishes  the  American  farmer 
with  the  bulk  of  his  potash,  she  may  not  do  so  much  longer. 
In  1911  an  indirect  potash  tax  was  levied  by  Germany  on  her 
best  customer,  the  United  States,  to  whom  1 5  million  dollars' 
worth  of  potash  had  been  sold  the  preceding  year.  This  led 
Americans  to  inquire  whether  potash  could  not  be  obtained  at 
home. 

Geologists  say  that  long  ages  ago  an  ocean  covered 
Germany,  that  the  waters  of  the  ocean  slowly  evaporated  and 
that  the  various  substances  in  the  sea  water  were  deposited 
on  the  ocean  bed  in  thick  layers.  The  deposits  thus  left  by  the 
evaporation  of  the  sea  water  gradually  became  hidden  by  sedi- 
ment and  soil,  and  lost  to  sight.  From  such  deposits,  potash 
is  obtained.  Geologists  tell  us  that  our  own  Western  States 
were  once  covered  by  an  oceanx  and  that  the  waters  evapo- 
rated and  disappeared  from  our  land  very  much  as  they  did 
from  Germany.  The  Great  Salt  Lake  of  Utah  is  a  relic  of 
such  an  ocean.  If  it  be  true  that  an  ocean  once  covered  our 
Western  States,  there  may  be  buried  deposits  of  potash  there, 
and  to-day  the  search  for  the  hidden  treasure  is  going  on  with 
the  energy  and  enthusiasm  characteristic  of  America. 

Another  probable  source  of  potash  is  seaweed.  The  sea 
is  a  vast  reservoir  of  potash,  and  seaweed,  especially  the 
giant  kelp,  absorbs  large  quantities  of  this  potash.  A  ton  of 
dried  kelp  (dried  by  sun  and  wind)  contains  about  500  pounds 
of  pure  potash.  The  kelps  are  abundant,  covering  thousands 
of  square  miles  in  the  Pacific  Ocean,  from  Mexico  to  the 
Arctic  Ocean. 


CHAPTER   XXVII 


SOUND 

249.  The  Senses.  All  the  information  which  we  possess 
of  the  world  around  us  comes  to  us  through  the  use  of  the 
senses  of  sight,  hearing,  taste,  touch,  and  smell.  Of  the  five 
senses,  sight  and  hearing  are  generally  considered  the  most 
valuable.  In  preceding  Chapters  we  studied  the  important 
facts  relative  to  light  and  the  power  of  vision  ;  it  remains  for 
us  to  study  Sound  as  we  studied  Light,  and  to  learn  what  we 

can  of  sound  and  the  power  to  hear. 

250.  How  Sound  is  Produced.  If 
one  investigates  the  source  of  any 
sound,  he  will  always  find  that  it  is 
due  to  motion  of  some  kind.  A  sud- 
den noise  is  traced  to  the  fall  of  an 
object,  or  to  an  explosion,  or  to  a  col- 
lision ;  in  fact,  is  due  to  the  motion 
of  matter.  A  piano  gives  out  sound 
whenever  a  player  strikes  the  keys 
and  sets  in  motion  the  various  wires 
within  the  piano ;  speech  and  song 
are  caused  by  the  motion  of  chest, 
vocal  cords,  and  lips. 

If  a  large  dinner  bell  is  rung,  its 
motion  or  vibration  may  be  felt  on 
touching  it  with  the  finger.  If  a  tuning 
fork  is  made  to  give  forth  sound  by  striking  it  against  the  knee, 
or  hitting  it  with  a  rubber  hammer,  and  is  then  touched  to  the 

266 


FlG.  164.  —  Sprays  of  water 
show  that  the  fork  is  in 
motion. 


SOUND   IS   CARRIED   BY  MATTER  267 

surface  of  water,  small  sprays  of  water  will  be  thrown  out, 
showing  that  the  prongs  of  the  fork  are  in  rapid  motion.  (A 
rubber  hammer  is  made  by  putting  a  piece  of  glass  tubing 
through  a  rubber  cork.) 

If  a  light  cork  ball  on  the  end  of  a  thread  is  brought  in 
contact  with  a  sounding  fork,  the  ball  does  not  remain  at  rest, 
but  vibrates  back  and  forth,  being 
driven  by  the  moving  prongs. 

These  simple  facts  lead  us  to  con- 
clude that  all  sound  is  due  to  the 
motion  of  matter,  and  that  a  sounding 
body  of  any  kind  is  in  rapid  motion. 

251.    Sound   is  carried  by  Matter.  FlG-  165. -The  bail  does  not 

0  J  remain  at  rest. 

In   most   cases    sound   reaches    the 

ear  through  the  air;  but  air  is  not  the  only  medium  through 
which  sound  is  carried.  A  loud  noise  will  startle  fish,  and 
cause  them  to  dart  away,  so  we  conclude  that  the  sound  must 
have  reached  them  through  the  water.  An  Indian  puts  his 
ear  to  the  ground  in  order  to  detect  distant  footsteps,  because 
sounds  too  faint  to  be  heard  through  the  air  are  comparatively 
clear  when  transmitted  through  the  earth.  A  gentle  tapping 
at  one  end  of  a  long  table  can  be  distinctly  heard  at  the 
opposite  end  if  the  ear  is  pressed  against  the  table ;  if  the 
ear  is  removed  from  the  wood,  the  sound  of  tapping  is  much 
fainter,  showing  that  wood  transmits  sound  more  readily  than 
air.  We  see  therefore  that  sound  can  be  transmitted  to  the 
ear  by  solids,  liquids,  or  gases. 

Matter  of  any  kind  can  transmit  sound  to  the  ear.  The 
following  experiments  will  show  that  matter  is  necessary  for 
transmission.  Attach  a  small  toy  bell  to  a  glass  rod  (Fig. 
1 66)  by  means  of  a  rubber  tube  and  pass  the  rod  through  one 
of  two  openings  in  a  rubber  cork.  Insert  the  cork  in  a  strong 
flask  containing  a  small  quantity  of  water  and  shake  the  bell, 


268 


SOUND 


noting  the  sound  produced.  Then  heat  the  flask,  allowing  the 
water  to  boil  briskly,  and  after  the  boiling  has  continued  for  a 
few  minutes  remove  the  flame  and  instantly 
close  up  the  second  opening  by  inserting  a 
glass  stopper.  Now  shake  the  flask  and 
note  that  the  sound  is  very  much  fainter 
than  at  first.  As  the  flask  was  warmed,  air 
was  rapidly  expelled ;  so  that  when  the 
flask  was  shaken  the  second  time,  less  air 
was  present  to  transmit  the  sound.  If  the 
glass  stopper  is  removed  and  the  air  is 
allowed  to  reenter  the  flask,  the  loudness 
of  the  sound  immediately  increases. 

Since  the  sound  of  the  bell  grows  fainter 
as  air  is  removed,  we  infer  that  there  would 
be  no  sound   if  all  the  air  were  removed 
FIG.  166  -  Sound  is  car-  from  the  flask  ;  that  is  to  say,  sound  can- 

ried  by  the  air.  .    ,  ., .     ,    , ,  . 

not  be  transmitted  through  empty  space 
or  a  vacuum.  If  sound  is  to  reach  our  ears,  it  must  be  through 
the  agency  of  matter,  such  as  wood,  water,  or  air,  etc. 

252.  How  Sound  is  transmitted  through  Air.  We  saw  in 
Section  250  that  sound  can  always  be  traced  to  the  motion 
or  vibration  of  matter.  It  is  impossible  to  conceive  of  an 
object  being  set  into  sudden  and  continued  motion  without 
disturbing  the  air  immediately  surrounding  it.  A  sounding 
body  always  disturbs  and  throws  into  vibration  the  air  around 
it,  and  the  air  particles  which  receive  motion  from  a  sounding 
body  transmit  their  motion  to  neighboring  particles,  these  in 
turn  to  the  next  adjacent  particles,  and  so  on  until  the  motion 
has  traveled  to  very  great  distances.  The  manner  in  which 
vibratory  motion  is  transmitted  by  the  atmosphere  must  be 
unusual  in  character,  since  no  motion  of  the  air  is  apparent, 
and  since  in  the  stillness .oi^night  when  "  not  a  breath  of  air  "  is 


HOW  SOUND  IS  TRANSMITTED   THROUGH  AIR     269 


FIG.  167.  —  Elastic 
balls. 


stirring,  the  shriek  of  a  railroad  whistle  miles  distant  may  be 
heard  with  perfect  clearness.     Moreover,  the  most  delicate 
notes  of  a  violin  can  be  heard  in  the  remotest  corners  of  a 
concert  hall,  when  not  the  slightest  motion 
of  the  air  can  be  seen  or  felt. 

In  our  study  of  the  atmosphere  we  saw 
that  air  can  be  compressed  and  rarefied ;  in 
other  words,  we  saw  that  air  is  very  elastic. 
It  can  be  shown  experimentally  that  when- 
ever an  elastic  body  in  motion  comes  in  con- 
tact with  a  body  at  rest,  the  moving  body 
transfers  its  motion  to  the  second  body 
and  then  comes  to  rest  itself.  Let  two  billiard  balls  be  sus- 
pended in  the  manner  indicated  in  Figure  167.  If  one  of  the 
balls  is  drawn  aside  and  is  then  allowed  to  fall  against  the  other, 
the  second  ball  is  driven  outward  to  practically  the  height  from 
which  the  first  ball  fell  and  the  first  ball  comes  to  rest. 

If  a  number  of  balls  are  arranged  in  line  as  in  Figure  168  or 
Figure  169,  and  the  end  ball  is  raised  and  then  allowed  to  fall, 
or  if  A  is  pushed  against  C,  the  last  ball  B  will  move  outward 
alone,  with  a  force  nearly  equal  to  that  originally  possessed 
by  A  and  to  a  distance  nearly  equal  to  that  through  which  A 
moved.  But  there  will  be  no  visible  motion  of  the  interven- 
ing balls.  The  force  of  the  moving 
ball  A  is  given  to  the  second  ball, 
and  the  second  ball  in  turn  gives 
the  motion  to  the  third,  and  so  on 
throughout  the  entire  number,  until 
B  is  reached.  But  B  has  no  ball  to 
168.  —  Suspended  billiard  give  its  motion  to,  hence  B  itself 

balls.  ' 

moves  outward,  and  moves  with  a 

force  nearly  equal  to  that  originally  imparted  by  A  and  to  a 
distance  nearly  equal  to  that  through  which  A  fell.     Motion 


2/0  SOUND 

at  A  is  transmitted  to  B  without  any  perceptible  motion  of 
the  balls  lying  between  these  points.     Similarly  the  particles 


o— oooo- 


I  / 

FlG.  169.  — Elastic  balls  transmit  motion. 

of  air  set  into  motion  by  a  sounding  body  impart  their  motion 
to  each  other,  the  motion  being  transmitted  onward  without 
any  perceptible  motion  of  the  air  itself.  When  this  motion 
reaches  the  ear,  it  sets  the  drum  of  the  ear  into  vibration,  and 
these  vibrations  are  in  turn  transmitted  to  the  auditory  nerves, 
which  interpret  the  motion  as  sound. 

253.  Why  Sound  dies  away  with  Distance.  Since  the  last  ball 
B  is  driven  outward  with  a  force  nearly  equal  to  that  possessed 
by  A,  it  would  seem  that  the  effect  on  the  ear  drum  should 
be  independent  of  distance  and  that  a  sound  should  be  heard 
as  distinctly  when  remote  as  when  near.  But  we  know  from 


— o 


\ 


71  ' 

FlG.  170.  —  When  a  ball  meets  more  than  one  ball,  it  divides  its  motion. 

experience  that  this  is  not  true,  because  the  more  distant  the 
source  of  sound,  the  fainter  the  impression ;  and  finally,  if 


VELOCITY  OF  SOUND  271 

the  distance  between  the  source  of  sound  and  the  hearer  be- 
comes too  great,  the  sound  disappears  entirely  and  nothing 
is  heard.  The  explanation  of  this  well-known  fact  is  found 
in  a  further  study  of  the  elastic  balls  (Fig.  170).  If  A  hits  two 
balls  instead  of  one,  the  energy  possessed  by  A  is  given  in 
part  to  one  ball,  and  in  part  to  the  other,  so  that  neither  ob- 
tains the  full  amount.  These  balls,  having  each  received  less 
than  the  original  energy,  have  less  to  transmit ;  each  of  these 
balls  in  turn  meets  with  others,  and  hence  the.  motion  be- 
comes more  and  more  distributed,  and  distant  balls  receive 
less  and  less  impetus.  The  energy  finally  given  becomes 
too  slight  to  affect  neighboring  balls,  and  the  system  comes 
to  rest.  This  is  what  occurs  in  the  atmosphere ;  a  moving 
air  particle  meets  not  one  but  many  adjacent  air  particles, 
and  each  of  these  receives  a  portion  of  the  original  energy  and 
transmits  a  portion.  When  the  original  disturbance  becomes 
scattered  over  a  large  number  of  air  particles,  the  energy  given 
to  any  one  air  particle  becomes  correspondingly  small,  and 
finally  the  energy  becomes  so  small  that  further  particles  are 
not  affected;  beyond  this  limit  the  sound  cannot  be  heard. 

If  an  air  particle  -transmitted  motion  only  to  those  air  par- 
ticles directly  in  line  with  it,  we  should  not  be  able  to  detect 
sound  unless  the  ear  were  in  direct  line  with  the  source. 
The  fact  that  an  air  particle  divides  its  motion  among  all 
particles  which  it  touches,  that  is,  among  those  on  the  sides 
as  well  as  those  in  front,  makes  it  possible  to  hear  sound  in  all 
directions.  A  good  speaker  is  heard  not  only  by  those 
directly  in  front  of  him,  but  by  those  on  the  side,  and  even 
behind  him. 

254.  Velocity  of  Sound.  The  transmission  of  motion  from 
particle  to  particle  does  not  occur  instantaneously,  but  re- 
quires time.  If  the  distance  is  short,  so  that  few  air  particles 
are  involved,  the  time  required  for  transmission  is  very  brief, 


2/2  SOUND 

and  the  sound  is  heard  at  practically  the  instant  it  is  made. 
Ordinarily  we  are  not  conscious  that  it  requires  time  for 
sound  to  travel  from  its  source  to  our  ears,  because  the  dis- 
tance involved  is  too  short.  At  other  times  we  recognize 
that  there  is  a  delay ;  for  example,  thunder  reaches  our  ears 
after  the  lightning  which  caused  the  thunder  has  completely 
disappeared.  If  the  storm  is  near,  the  interval  of  time 
between  the  lightning  and  the  thunder  is  brief,  because  the 
sound  does  not  have  far  to  travel ;  if  the  storm  is  distant, 
the  interval  is  much  longer,  corresponding  to  the  greater  dis- 
tance through  which  the  sound  travels.  Sound  does  not 
move  instantaneously,  but  requires  time  for  its  transmission. 
The  report  of  a  distant  cannon  is  heard  after  the  flash  and 
smoke  are  seen ;  the  report  of  a  near  cannon  is  heard  the  in- 
stant the  flash  is  seen. 

The  speed  with  which  sounds  travels  through  the  air,  or  its 
velocity,  was  first  measured  by  noting  the  interval  (54.6 
seconds)  -which  elapsed  between  the  flash  of  a  cannon  and 
the  sound  of  the  report.  The  distance  of  the  cannon  from 
the  observer  was  measured  and  found  to  be  61,045  feet>  and 
by  dividing  this  distance  by  the  number  of  seconds,  we  find 
that  the  distance  traveled  by  sound  in  one  second  is  ap- 
proximately 1 1 1 8  feet. 

High  notes  and  low  notes,  soft  notes  and  shrill  notes,  all 
travel  at  the  same  rate.  If  bass  notes  traveled  faster  or 
slower  than  soprano  notes,  or  if  the  delicate  tones  of  the 
violin  traveled  faster  or  slower  than  the  tones  of  a  drum, 
music  would  be  practically  impossible,  because  at  a  distance 
from  the  source  of  sound  the  various  tones  which  should 
be  in  unison  would  be  out  of  time  —  some  arriving  late, 
some  early. 

255.  Sound  Waves.  Practically  every  one  knows  that  a 
hammock  hung  with  long  ropes  swings  or  vibrates  more  slowly 


SOUND    WAVES 


273 


T 

FIG.  171.  —  The  two  hammocks  swing  differently. 


than  one  hung  with  short  ropes,  and  that  a  stone  suspended 
by  a  long  string  swings  more  slowly  than  one  suspended  by  a 
short  string.  No  two 
rocking  chairs  vibrate 
in  the  same  way  unless  t'v 

they  are  exactly  alike  in    \  :*$£\  ^«— ^ , ,  M ,  -r-Tffl*^r     /it*) 

shape,  size,  and  material. 
An  object  when  dis- 
turbed vibrates  in  a 
manner  peculiar  to  itself, 
the  vibration  being  slow, 
as  in  the  case  of  the 
long-roped  swing,  or 
quick,  as  in  the  case  of  the  short-roped  swing.  The  time 
required  for  a  single  swing  or  vibration  is  called  the  period  of 
the  body,  and  everything  that  can  vibrate  has  a  characteristic 
period.  Size  and  shape  determine  to  a  large  degree  the 
period  of  a  body ;  for  example,  a  short,  thick  tuning  fork 
vibrates  more  rapidly  than  a  tall  slender  fork. 
Some  tuning  forks  when  struck  vibrate  so 
rapidly  that  the  prongs  move  back  and  forth 
more  than  5000  times  per  second,  while  other 
tuning  forks  vibrate  so  slowly  that  the  vibra- 
tions do  not  exceed  50  per  second.  In  either 
case  the  distance  through  which  the  prongs 
move  is  very  small  and  the  period  is  very 
short,  so  that  the  eye  can  seldom  detect  the 
movement  itself.  That  the  prongs  are  in  mo- 
tion, however,  is  seen  by  the  action  of  a  pith 
ball  when  brought  in  contact  with  the  prongs 
(see  Section  250). 
The  disturbance  created  by  a  vibrating  body  is  called  a 
wave. 

CL.   GEN.    SCI. —  1 8 


FlG.  172.  — The 
pitch  given  out 
by  a  fork  de- 
pends upon  its 
shape. 


274 


SOUND 


256.   Waves.     While    the    disturbance   which    travels   out 
from  a  sounding  body  is  commonly  called  a  wave,  it  is  by  no 
means  like  the  type  of  wave  best  known 
X    <***  to  us,  namely,  the  water  wave. 

If   a .  closely  coiled   heavy  wire  is  sus- 
V^  pended  as  in  Figure  173  and  the  weight  is 

drawn  down  and  then  released,  the  coil 
will  assume  the  appearance  shown ;  there 
is  clearly  an  overcrowding  or  conden- 
sation in  some  places,  and  a  spreading 
out  or  rarefaction  in  other  places.  The 
pulse  of  condensation  and  rarefaction 
which  travels  the  length  of  the  wire  is 
called  a  wave,  although  it  bears  little  or 
no  resemblance  to  the  familiar  water  wave. 
Sound  waves  are  similar  to  the  waves 
formed  in  the  stretched  coil. 

Sound  waves  may  be  said  to  consist  of  a 
series  of  condensations   and  rarefactions, 
and  the  distance  between  two  consecutive 
condensations  and  rarefactions  may  be  de- 
/  fined  as  the  wave  length. 

f  257.   How  One  Sounding  Body  produces 

A  Sound  in  Another  Body.     In  Section   255 

*B  we  saw    that  any  object   when    disturbed 

FIG.  173.  —  Waves  in  a   vibrates  in  a  manner  peculiar  to  itself, — 

coiled  wire. 

its  natural  period,  —  a  long-roped  ham- 
mock vibrating  slowly  and  a  short-roped  hammock  vibrating 
rapidly.  From  observation  we  learn  that  it  requires  but  little 
force  to  cause  a  body  to  vibrate  in  its  natural  period.  If  a 
sounding  body  is  near  a  body  which  has  the  same  period  as 
itself,  the  pulses  of  air  produced  by  the  sounding  will,  although 
imperceptibly  small,  set  the  second  body  into  motion  and  cause 


SOUNDING  BODIES 


275 


it  to  make  a  faint  sound.  When  a  piano  is  being  played,  we  are 
often  startled  to  find  that  a  window  pane  or  an  ornament  re- 
sponds to  some  note  of  the  piano.  If  two  tuning  forks  of  ex- 
actly identical  periods  (that  is,  of  the  same  frequency)  are 
placed  on  a  table  as  in 
Figure  174,  and  one  is 
struck  so  as  to  give  forth 
a  clear  sound,  the  second 
fork  will  likewise  vibrate, 
even  though  the  two 
forks  may  be  separated 
by  several  feet  of  air. 
We  can  readily  see  that 
the  second  fork  is  in 
motion,  although  it  has 
not  been  struck,  because 
it  will  set  in  motion  a  pith  ball  suspended  beside  it;  at  first 
the  pith  ball  does  not  move,  then  it  moves  slightly,  and 
finally  bounces  rapidly  back  and  forth.  If  the  periods 
of  the  two  forks  are  not  identical,  but  differ  in  the  slight- 
est degree,  the  second  fork  will  not  respond  to  the  first 
fork,  no  matter  how  long  or  how  loud  the  sound  of  the  first 
fork.  If  we  suppose  that  the  fork  vibrates  256  times  each 
second,  then  256  gentle  pulses  of  air  are  produced  each 
second,  and  these,  traveling  outward  through  the  air,  reach 
the  silent  fork  and  tend  to  set  it  in  motion.  A  single  pulse 
of  air  could  not  move  the  solid,  heavy  prongs,  but  the  accu- 
mulated action  of  256  vibrations  per  second  soon  makes  itself 
felt,  and  the  second  fork  begins  to  vibrate,  at  first  gently, 
then  gradually  stronger,  and  finally  an  audible  tone  is  given 
forth. 

The  cumulative  power  of  feeble  forces  acting  frequently  at 
definite  intervals  is  seen  in  many  ways  in  everyday  life.     A 


276  SOUND 

small  boy  can  easily  swing  a  much  larger  boy,  provided  he 
gives  the  swing  a  gentle  push  in  the  right  direction  every 
time  it  passes  him.  But  he  must  be  careful  to  push  at  the 
proper  instant,  since  otherwise  his  effort  does  not  count  for 
much ;  if  he  pushes  forward  when  the  swing  is  moving  back- 
ward, he  really  hinders  the  motion  ;  if  he  waits  until  the  swing 
has  moved  considerably  forward,  his  push  counts  for  little. 
He  must  push  at  the  proper  instant ;  that  is,  the  way  in  which 
his  hand  moves  in  giving  the  push  must  correspond  exactly 
with  the  way  in  which  the  swing  would  naturally  vibrate.  A 
very  striking  experiment  can  be  made  by  suspending  from 
the  ceiling  a  heavy  weight  and  striking  this  weight  gently  at 
regular,  properly  timed  intervals  with  a  small  cork  hammer. 
Soon  the  pendulum,  or  weight,  will  be  set  swinging. 

258.   Borrowed    Sound.     Picture    frames    and    ornaments 
sometimes  buzz  and  give  forth  faint  murmurs  when  a  piano 
or  organ  is  played.     The  waves  sent  out  by  a 
sounding  body  fall  upon  all  surrounding  ob- 
jects  and   by   their  repeated  action  tend  to 
throw    these    bodies    into    vibration.     If   the 
period  of  any  one  of  the  objects  corresponds 
with   the    period   of  the  sounding  body,   the 
gentle  but  frequent  impulses  affect  the  object, 
FIG.     175.— The    which  responds  by  emitting  a  sound.     If,  how- 
hollow    wooden    ever,  the  periods  do  not  correspond,  the  action 

box      reenforces 

the  sound.  of  the  sound  waves  is  not  sufficiently  powerful 

to  throw  the  object  into  vibration,  and  no 
sound  is  heard.  Bodies  which  respond  in  this  way  are  said 
to  be  sympathetic  and  the  response  produced  is  called  res- 
onance. Seashells  when  held  to  the  ear  seem  to  contain  the 
roar  of  the  sea ;  this  is  because  the  air  within  the  shell  is  set 
into  sympathetic  vibrations  by  some  external  tone.  If  the 
seashell  were  held  to  the  ear  in  an  absolutely  quiet  room,  no 


ECHO  277 

sound  would  be  heard,  because  there  would  be  no  external 
forces  to  set  into  vibration  the  air  within  the  shell. 

Tuning  forks  do  not  produce  strong  tones  unless  mounted 
on  hollow  wooden  boxes  (Fig.  175),  whose  size  and  shape 
are  so  adjusted  that  resonance  occurs  and  strengthens'  the 
sound.  When  a  human  being  talks  or  sings,  the  air  within 
the  mouth  cavity  is  thrown  into  sympathetic  vibration  and 
strengthens  the  otherwise  feeble  tone  of  the  speaker. 

259.  Echo.  If  one  shouts  in  a  forest,  the  sound  is  some- 
times heard  a  second  time  a  second  or  two  later.  This  is  be- 
cause sound  is  reflected  when  it  strikes  a  large  obstructing 
surface.  If  the  sound  waves  resulting  from  the  shout  meet  a 
cliff  or  a  mountain,  they  are  reflected  back,  and  on  reaching 
the  ear  produce  a  later  sensation  of  sound. 

By  observation  it  has  been  found  that  the  ear  cannot  dis- 
tinguish sounds  which  are  less  than  one  tenth  of  a  second 
apart ;  that  is,  if  two  sounds  follow  each  other  at  an  interval 
less  than  one  tenth  of  a  second,  the  ear  recognizes  not  two 
sounds,  but  one.  This  explains  why  a  speaker  can  be  heard 
better  indoors  than  in  the  open  air.  In  the  average  building, 
the  walls  are  so  close  that  the  reflected  waves  have  but  a  short 
distance  to  travel,  and  hence  reach  the  ear  at  practically  the 
same  time  as  those  which  come  directly  from  the  speaker. 
In  the  open,  there  are  no  reflecting  walls  or  surfaces,  and  the 
original  sound  has  no  reinforcement  from  reflection. 

If  the  reflected  waves  reach  the  ear  too  late  to  blend  with  the 
original  sound,  that  is,  come  later  than  one  tenth  of  a  second 
after  the  first  impression,  an  echo  is  heard.     What  we  call 
the  rolling  of  thunder  is  really  the  reflection  and  re-reflection  , 
of  the  original  thunder  from  cloud  and  cliff. 

Some  halls  are  so  large  that  the  reflected  sounds  cause  a 
confusion  of  echoes,  but  this  difficulty  can  be  lessened  by 
hanging  draperies,  which  break  the  reflection. 


2/8  SOUND 

260.  Motion  does  not  always  produce  Sound.      While  we 
know  that  all  sound  can  be  traced  to  motion,  we  know  equally 
well  that  motion  does  not  always  produce  sound.     The  ham- 
mock swinging  in  the  breeze  does  not  give  forth  a  sound ; 
the  flag  floating  in  the  air  does  not  give  forth  a  sound  unless 
blown  violently  by  the   wind  ; .  a  card  moved  slowly  through 
the  air  does  not  produce  sound,   but  if  the  card  is  moved 
rapidly  back  and  forth,  a  sound  becomes  audible. 

Motion,  in  order  to  produce  sound,  must  be  rapid ;  a  ball 
attached  to  a  string  and  moved  slowly  through  the  air  produces 
no  sound,  but  the  same  ball,  whirled  rapidly,  produces  a  dis- 
tinct buzz,  which  becomes  stronger  and  stronger  the  faster 
the  ball  is  whirled. 

261.  Noise  and  Music.     When  the  rapid  motions   which 
produce  sound  are  irregular,  we  hear  noise  ;  when  the  motions 
are  regular  and  definite,  we  have  a  musical  tone ;  the  rattling 
of  carriage  wheels  on  stones,  the  roar  of  waves,  the  rustling 
of  leaves  are  noise,  not  music.     In  all  these  illustrations  we 
have   rapid    but  irregular   motion ;  no  two  stones  strike  the 
wheel  in  exactly  the  same  way,  no  two  waves  produce  pulses 
in  the  air  of  exactly  the  same  character,  no  two  leaves  rustle 
in  precisely  the  same  way.     The  disturbances  which  reach 
the  air  from  carriage,  waves,  and  leaves  are  irregular  both  in 
time  and  strength,  and  irritate  the  ear,  causing  the  sensa- 
tion which  we  call  noise. 

The  tuning  fork  is  musical.  Here  we  have  rapid,  regular 
motion ;  vibrations  follow  each  other  at  perfectly  definite 
intervals,  and  the  air  disturbance  produced  by  one  vibration  is 
exactly  like  the  disturbance  produced  by  a  later  vibration. 
The  sound  waves  which  reach  the  ear  are  regular  in  time  and 
kind  and  strength,  and  we  call  the  sensation  music. 

To  produce  noise  a  body  must  vibrate  in  such  a  way  as  to 
give  short,  quick  shocks  to  the  air ;  to  produce  music  a  body 


NOISE  AND  MUSIC  279 

must  not  only  impart  short,  quick  shocks  to  the  air,  but 
must  impart  these  shocks  with  unerring  regularity  and 
strength.  A  flickering  light  irritates  the  eye;  a  flickering 
sound  or  noise  irritates  the  ear ;  both  are  painful  because  of 
the  sudden  and  abrupt  changes  in  effect  which  they  cause, 
the  former  on  the  eye,  the  latter  on  the  ear. 

The  only  thing  essential  for  the  production  of  a  musical 
sound  is  that  the  waves  which  reach  the  ear  shall  be  rapid 
and  regular ;  it  is  immaterial  how  these 
waves  are  produced.  If  a  toothed  wheel 
is  mounted  and  slowly  rotated,  and  a  stiff 
card  is  held  against  the  teeth  of  the  wheel, 
a  distinct  tap  is  heard  every  time  the  card 
strikes  the  wheel.  But  if  the  wheel  is  ro- 
tated rapidly,  the  ear  ceases  to  hear  the 
various  taps  and  recognizes  a  deep  contin- 
uous musical  tone.  The  blending  of  the 

.     , .    .  ,       ,  .  ,        .    ,  FIG.  176.  —  A  rotating 

individual  taps,  occurring  at  regular  inter-  disk, 

vals,    has    produced    a   sustained    musical 
tone.     A  similar  result  is  obtained  if  a  card  is  drawn  slowly 
and  then  rapidly  over  the  teeth  of  a  comb. 

That  musical  tones  are  due  to  a  succession  of  regularly 
timed  impulses  is  shown  most  clearly  by  means  of  a  rotating 
disk  on  which  are  cut  two  sets  of  holes,  the  outer  set  equally 
spaced,  and  the  inner  set  unequally  spaced  (Fig.  176). 

If,  while  the  disk  is  rotating  rapidly,  a  tube  is  held  over 
the  outside  row  and  air  is  blown  through  the  tube,  a  sus- 
tained musical  tone  will  be  heard.  If,  however,  the  tube  is 
held,  during  the  rotation  of  the  disk,  over  the  inner  row  of 
unequally  spaced  holes,  the  musical  tone  disappears,  and  a 
series  of  noises  take  its  place.  In  the  first  case,  the  separate 
puffs  of  air  followed  each  other  regularly  and  blended  into 
one  tone ;  in  the  second  case,  the  separate  puffs  of  air  followed 


280  SOUND 

each  other  at  uncertain  and  irregular  intervals  and  the  result 
was  noise. 

Sound  possesses  a  musical  quality  only  when  the  waves  or 
pulses  follow  each  other  at  absolutely  regular  intervals. 

262.  The  Effect  of  the  Rapidity  of  Motion  on  the  Musical 
Tone  Produced.     If  the  disk  is  rotated  so  slowly  that  less 
than  1 6  puffs  of  air  are  produced  in  one  second,  only  sepa- 
rate puffs  are  heard,  and  a  musical  tone  is  lacking ;  if,  on 
the  other  hand,  the  disk  is  rotated  in  such  a  way  that  16  puffs 
or  more  are  produced  in  one  second,  the  separate  puffs  will 
blend  together  to  produce  a  continuous  musical  note  of  very 
low  pitch.     If  the  speed  of  the  disk  is  increased  so  that  the 
puffs  become  more  frequent,  the  pitch  of  the  resulting  note 
rises ;  and  at  very  high  speeds  the  notes  produced  become 
so  shrill  and  piercing  as  to  be  disagreeable  to  the  ear.    If  the 
speed  of  the  disk  is  lessened,  the  pitch  falls  correspondingly ; 
and  if  the  speed  again  becomes  so  low  that  less  than  16  puffs 
are  formed  per  second,  the  sustained  sound  disappears  and 
a  series  of  intermittent  noises  is  produced. 

263.  The  Pitch  of  a  Note.     By  means   of   an   apparatus 
called  the  siren,  it  is  possible  to  calculate  the  number  of  vibra- 
tions producing  any  given  musical  note,  such,  for  example,  as 
middle  C  on  the  piano.     If  air  is  forced  continuously  against 
the  disk  as  it  rotates,  a  series  of  puffs  will  be  heard  (Fig.  177). 

If  the  disk  turns  fast  enough,  the  puffs  blend  into  a  musical 
sound,  whose  pitch  rises  higher  and  higher  as  the  disk  moves 
faster  and  faster,  and  produces  more  and  more  puffs  each 
second. 

The  instrument  is  so  constructed  that  clockwork  at  the 
top  registers  the  number  of  revolutions  made  by  the  disk  in 
one  second.  The  number  of  holes  in  the  disk  multiplied  by 
the  number  of  revolutions  a  second  gives  the  number  of 
puffs  of  air  produced  in  one  second.  If  we  wish  to  find  the 


THE  PITCH  OF  A  NOTE 


281 


number  of  vibrations  which  correspond  to  middle  C  on  the 
piano,  we  increase  the  speed  of  the  disk  until  the  note  given 
forth  by  the  siren  agrees  with  middle  C  as  sounded  on  the 
piano,  as  nearly  as  the  ear  can  judge;  we  then  calculate  the 
number  of  puffs  of  air  which  took  place  each  second  at  that 
particular  speed  of  the  disk.  In  this  way  we  find  that  middle 
C  is  due  to  about  256  vibrations  per  second ;  that  is,  a  piano 


FIG.  177.  —  A  siren. 


string  must  vibrate  256  times  per  second  in  order  for  the  re- 
sultant note  to  be  of  pitch  middle  C.  In  a  similar  manner  we 
determine  the  following  frequencies:  — 


do 

re 

mi 

fa 

sol 

la 

si 

do 

C 

D 

E 

F 

G 

A 

B 

C' 

256 

288 

320 

341 

384 

427 

480 

$12 

The  pitch  of  pianos,  from  the  lowest  bass  note  to  the  very 
highest  treble,  varies  from  27  to  about  3500  vibrations  per 
second.  No  human  voice,  however,  has  so  great  a  range  of 
tone;  the  highest  soprano  notes  of  women  correspond  to 


282  SOUND 

but  I  OCX)  vibrations  a  second,  and  the  deepest  bass  of  men 
falls  but  to  80  vibrations  a  second. 

While  the  human  voice  is  limited  in  its  production  of  sound, 
—  rarely  falling  below  80  vibrations  a  second  and  rarely  ex- 
ceeding 1000  vibrations  a  second,  —  the  ear  is  by  no  means  lim- 
ited to  that  range  in  hearing.  The  chirrup  of  a  sparrow,  the 
shrill  sound  of  a  cricket,  and  the  piercing  shrieks  of  a  locomo- 
tive are  due  to  far  greater  frequencies,  the  number  of  vibra- 
tions at  times  equaling  38,000  per  second. 

264.  The  Musical  Scale.  When  we  talk,  the  pitch  of  the 
voice  changes  constantly  and  adds  variety  and  beauty  to  con- 
versation ;  a  speaker  whose  tone,  or  pitch,  remains  too  con- 
stant is  monotonous  and  dull,  no  matter  how  brilliant  his 
thoughts  may  be. 

While  the  pitch  of  the  voice  changes  constantly,  the  changes 
are  normally  gradual  and  slight,  and  the  different  tones 
merge  into  each  other  imperceptibly.  In  music,  however, 
there  is  a  well-defined  interval  between  even  consecutive 
notes ;  for  example,  in  the  musical  scale,  middle  C  (do)  with 
256  vibrations  is  followed  by  D  (re)  with  288  vibrations,  and 
the  interval  between  these  notes  is  sharp  and  well  marked, 
even  to  an  untrained  ear.  The  interval  between  two  notes  is 
defined  as  the  ratio  of  the  frequencies  ;  hence,  the  interval  be- 
tween C  and  D  (do  and  re)  is  |||,  or  |.  Referring  to  Sec- 
tion 263,  we  see^that  the  interval  between  C  and  E  is  f-f  j},  or 
f ,  and  the  interval  between  C  and  C'  is  |||,  or  2 ;  the  in- 
terval between  any  note  and  its  octave  is  2. 

The  successive  notes  in  one  octave  of  the  musical  scale  are 
related  as  follows  :  — 

Key  of  C  CDEFGABC' 

No.  of  vibra- 
tions per  sec.  256     288     320     341      384     427     480     512 
Interval  f         f         I        1         I       Y        2 


THE  MUSICAL  SCALE 


283 


@ 


The  intervals  of  F  and  A  are  not  strictly  |  and  f ,  but  are 
nearly  so;  if  F  made  341.3  vibrations  per  second  instead  of 
341;  and  if  A  made  426.6  instead  of  427,  then  the  intervals 
would  be  exactly  f  and  |.  Since  the  real  difference  is  so  slight, 
we  can  assume  the  simpler  ratios  without  appreciable  error. 

Any  eight  notes  whose  frequencies  are  in  the  ratio  of  |,  f , 
etc.,  will  when  played  in  succession  give  the  familiar  musical 
scale;  for  example,  the  deepest  bass  voice  starts  a  musical 
scale  whose  notes  have  the  frequencies  80,  90,  100,  107,  120, 
133,  150,  1 60,  but  the  intervals  here  are  identical  with  those 
of  a  higher  scale ;  the  interval  between  C  and  D,  80  and  90, 
is  f,  just  as  it  was  before  when  the  frequencies  were  much 
greater;  that  is,  256  and  288.  In  singing  "Home,  Sweet 
Home,"  for  ex- 
ample, a  bass  voice  i  •£  £  ^  j-  ^  J.  4.  £  A  &.  £ 

may    start    with    a       '     *   \\  ri  *  f  \  v       *\  m  _~^~~ i —   — i 

.,      .          ,       /r>?  Jl  F  r  I  I  I    f  r  Ir-P  r»l   ^=J  A 
note   vibrating  only 

132  times  a  second  ; 

while  a   tenor  may 

start    at    a    higher 

pitch,    with   a   note 

vibrating  198  times 

per  second,    and    a 

soprano    would 

probably      take      a 

much  higher  range 

still,  with  an  initial 

frequency     of     528 

vibrations  per  second.     But  no  matter  where  the  voices  start, 

the  periods  are  always  identical.     The  air  as  sung  by  the 

bass  voice  would  be  represented  by  A.     The  air  as  sung  by 

the  tenor  voice  would  be  represented  by  B.     The  air  as  sung 

by  the  soprano  voice  would  be  represented  by  C. 


P 


132  vibrations 


'  i  If  If 


if 


iqs 


.bra.t,ons 

f 


ii 


3F 

sHffffl  f 

*•    ^        — 


5Z8  vibrations 

FIG.  178.  —  A  song  as  sung  by  three  voices  of 
different  pitch. 


CHAPTER   XXVIII 

MUSICAL   INSTRUMENTS 

265.  Musical  instruments  maybe  divided  into  three  groups 
according    to  the  different  ways  in  which   their    tones   are 
produced :  — 

First.  The  stringed  instruments  in  which  sound  is  pro- 
duced by  the  vibration  of  stretched  strings,  as  in  the  piano, 
violin,  guitar,  mandolin. 

Second.  The  wind  instruments  in  which  sound  is  produced 
by  the  vibrations  of  definite  columns  of  air,  as  in  the  organ, 
flute,  cornet,  trombone. 

Third.  The  percussion  instruments,  in  which  sound  is 
produced  by  the  motion  of  stretched  membranes,  as  in  the 
drum,  or  by  the  motion  of  metal  disks,  as  in  the  tambourines 
and  cymbals. 

266.  Stringed  Instruments.     If  the  lid  of  a  piano  is  opened, 
numerous  wires  are  seen  within  ;  some  long,  some  short,  some 
coarse,  some  fine.     Beneath  each  wire  is  a  small  felt  ham- 
mer connected  with  the  keys  in  such  a  way  that  when  a  key 
is  pressed,  a  string  is  struck  by  a  hammer  and  is  thrown  into 
vibration,  thereby  producing  a  tone. 

If  we  press  the  lowest  key,  that  is,  the  key  giving  forth  the 
lowest  pitch,  we  see  that  the  longest  wire  is  struck  and  set 
into  vibration ;  if  we  press  the  highest  key,  that  is,  the  key 
giving  the  highest  pitch,  we  see  that  the  shortest  wire  is 
struck.  In  addition,  it  is  seen  that  the  short  wires  which 

284 


STRINGED   INSTRUMENTS 


285 


produce  the  high  tones  are  fine,  while  the  long  wires  which 
produce  the  low  tones  are  coarse.  The  shorter  and  finer  the 
wire,  the  higher  the  pitch  of  the  tone  produced.  The  longer 
and  coarser  the  wire,  the  lower  the  pitch  of  the  tone  produced. 
The  constant  striking  of  the  hammers  against  the  strings 
stretches  and  loosens  them  and  alters  their  pitch ;  for  this 


FIG.  179.  —  Piano  wires  seen  from  the  back. 

reason  each  string  is  fastened  to  a-  screw  which  can  be  turned 
so  as  to  tighten  the  string  or  to  loosen  it  if  necessary.  The 
tuning  of  the  piano  is  the  adjustment  of  the  strings  so  that 
each  shall  produce  a  tone  of  the  right  pitch.  When  the  strings 
are  tightened,  the  pitch  rises ;  when  the  strings  are  loosened, 
the  pitch  falls. 

What  has  been  said  of  the  piano  applies  as  well  to  the 
violin,  guitar,  and  mandolin.  In  the  latter  instruments  the 
strings  are  few  in  number,  generally  four,  as  against  eighty- 


286  MUSICAL   INSTRUMENTS 

eight  in  the  piano ;  the  hammer  of  the  piano  is  replaced  in 
the  violin  by  the  bow,  and  in  the  guitar  by  the  fingers ;  vary- 
ing pitches  on  any  one  string  are  obtained  by  sliding  a  finger 
of  the  left  hand  along  the  wire,  and  thus  altering  its  length. 
Frequent  tuning  is  necessary,  because  the  fine  adjustments 
are  easily  disturbed.  The  piano  is  the  best  protected  of  all 


mmmmmi  i  iTimTriririi"i)     HHiFi    fc 


FIG.  180.  —  Front  view  of  an  open  piano. 

the  stringed  instruments,  being  inclosed  by  a  heavy  frame- 
work, even  when  in  use. 

267.  Strings  and  their  Tones.  Fasten  a  violin  string  to  a 
wooden  frame  or  box,  as  shown  in  Figure  181,  stretching  it 
by  means  of  some  convenient  weight ;  then  lay  a  yard- 
stick along  the  box  in  order  that  the  lengths  may  be  deter- 
mined accurately.  If  the  stretched  string  is  plucked  with 
the  fingers  or  bowed  with  the  violin  •  bow,  a  clear  musical 
sound  of  definite  pitch  will  be  produced.  Now  divide  the 


STRINGS  AND   THEIR   TONES 


287 


FlG.  181.  —  The  length  of  a  string  influences 
the  pitch. 


string  into  two  equal  parts  by  inserting  the  bridge  midway 
between  the  two  ends ;  and  pluck  either  half  as  before.  The 
note  given  forth  is  of  a  de- 
cidedly higher  pitch/ and  if 
by  means  of  the  siren  we 
compare  the  pitches  in  the 
two  cases,  we  find  that  the 
note  sounded  by  the  half  wire 
is  the  octave  of  the  note 
sounded  by  the  entire  wire ; 
the  frequency  has  been  doubled  by  halving  the  length.  If 
now  the  bridge  is  placed  so  that  the  string  is  divided  into 
two  unequal  portions  such  as  I  :  3  and  2  :  3,  and  the  shorter 
portion  is  plucked,  the  pitch  will  be  still  higher ;  the  shorter 
the  length  plucked,  the  higher  the  pitch  produced.  This  mov- 
able bridge  corresponds  to  the  finger  of  the  violinist;  the  finger 
slides  back  and  forth  along  the  string,  thus  changing  the 
length  of  the  bowed  portion  and  producing  variations  in  pitch. 
If  there  were  but  one  string,  only  one  pitch  could  be 


FIG.  182.  —  Only  one  half  of  the  string  is  bowed,  but  both  halves  vibrate. 

sounded  at  any  one  time ;    the  additional  strings  of  the  violin 
allow  of  the  simultaneous  production  of  several  tones. 


288 


MUSICAL  INSTRUMENTS 


268.  The  Freedom  of  a  String.  Some  stringed  instruments 
give  forth  tones  which  are  clear  and  sweet,  but  withal  thin 
and  lacking  in  richness  and  fullness.  The  tones  sounded  by 
two  different  strings  may  agree  in  pitch  and  loudness  and  yet 
produce  quite  different  effects  on  the  ear,  because  in  one 
case  the  tone  may  be  much  more  pleasing  than  in  the  other. 
The  explanation  of  this  is,  that  a  string  may  vibrate  in  a 
number  of  different  ways. 

Touch  the  middle  of  a  wire  with  the  finger  or  a  pencil  (Fig. 
182),  thus  separating  it  into  two  portions  and  draw  a  violin 


FIG.  183. — The  string  vibrates  in  three  portions. 

bow  across  the  center  of  either  half.  Only  one  half  of  the 
entire  string  is  struck,  but  the  motion  of  this  half  is  imparted 
to  the  other  half  and  throws  it  into  similar  motion,  and  if  a 
tiny  A-shaped  piece  of  paper  or  rider  is  placed  upon  the  un- 
bowed half,  it  is  hurled  off. 

If  the  wire  is  touched  at  a  distance  of  one  third  its  length 
and  a  bow  is  drawn  across  the  middle  of  the  smaller  portion, 
the  string  will  vibrate  in  three  parts ;  we  cannot  always  see 
these  various  motions  in  different  parts  of  the  string,  but  we 
know  of  their  existence  through  the  action  of  the  riders. 


THE  FREEDOM  OF  A  STRING  289 

Similarly,  touching  the  wire  one  fourth  of  its  length  from 
an  end  makes  it  vibrate  in  four  segments;  touching  it  one 
fifth  of  its  length  makes  it  vibrate  in  five  segments. 

In  the  first  case,  the  string  vibrated  as  a  whole  string  and 
also  as  two  strings  of  half  the  length ;  hence,  three  tones 
must  have  been  given  out,  one  tone  due  to  the  entire  strings 
and  two  tones  due  to  the  segments.  But  we  saw  in  Section 
267  that  halving  the  length  of  a  string  doubles  the  pitch  of 
the  resulting  tone,  and  produces  the  octave  of  the  original 
tone;  hence  a  string  vibrating  as  in  Figure  183  gives  forth 


FlG.  184.  —  When  a  string  vibrates  as  a  whole,  it  gives  out  the  fundamental  note. 

three  tones,  one  of  which  is  the  fundamental  tone  of  the 
string,  and  two  of  which  are  the  octave  of  the  fundamental 
tone.  Hence,  the  vibrating  string  produces  two  sensations, 
that  of  the  fundamental  note  and  of  its  octave. 

When  a  string  is  plucked  in  the  middle  without  being  held, 
it  vibrates  simply  as  a  whole  (Fig.  184),  and  gives  forth  but  one 
note ;  this  is  called  the  fundamental.  If  the  string  is  made  to 
vibrate  in  two  parts,  it  gives  forth  two  notes,  the  fundamental, 
and  a  note  one  octave  higher  than  the  fundamental;  this  is 
called  the  first  overtone.  When  the  string  is  made  to  move 
as  in  Figure  183,  three  distinct  motions  are  called  forth,  the 

CL.   GEN.    SCI. —  19 


290 


MUSICAL  INSTRUMENTS 


Tu,rxl  a/nerft  aJ 


motion  of  the  entire  string,  the  motion  of  the  portion  plucked, 
and  the  motion  of  the  remaining  unplucked  portion  of  the 
string.  Here,  naturally,  different  tones  arise,  corresponding 
to  the  different  modes  of  vibration.  The  note  produced  by 
the  vibration  of  one  third  of  the  original  string  is  called  the 
second  overtone. 

The  above  experiments  show  that  a  string  is  able  to  vibrate 
in  a  number  of  different  ways  at  the  same  time,  and  to  emit 

simultaneously  a  num- 
ber of  different  tones ; 
also  that  the  resulting 
complex  sound  consists 
of  the  fundamental 
and  one  or  more  over- 
tones, arfd  that  the 
number  of  overtones 
present  depends  upon 
how  and  where  the 
string  is  plucked. 

269.  The  Value  of 
Overtones.  The  pres- 
ence of  overtones  de- 
termines the  quality 


^th  Overtone 


FIG.   185.— A  string   can  vibrate  in  a   number  of     of  the  SOlind  produced, 
different  ways  simultaneously,  and  can  produce      ,  r      ,  ., 

different  notes  simultaneously.  If    the    String   Vibrates 

as  a  whole  merely,  the 

tone  given  out  is  simple,  and  seems  dull  and  characterless. 
If,  on  the  other  hand,  it  vibrates  in  such  a  way  that 
overtones  are  present,  the  tone  .given  forth  is  full  and  rich 
and  the  sensation  is  pleasing.  A  tuning  fork  cannot  vi- 
brate in  more  than  one  way,  and  hence  has  no  overtones, 
and  its  tone,  while  clear  and  sweet,  is  far  less  pleasing  than 
the  same  tone  produced  by  a  violin  or  piano.  The  un- 


THE  INDIVIDUALITY  OF  INSTRUMENTS  291 

trained  ear  is  not  conscious  of  overtones  and  recognizes 
only  the  strong  dominant  fundamental.  The  overtones  blend 
in  with  the  fundamental  and  are  so  inconspicuously  present 
that  we  do  not  realize  their  existence ;  it  is  only  when  they 
are  absent  that  we  become  aware  of  the  beauty  which  they 
add  to  the  music.  A  song  played  on  tuning  forks  instead 
of  on  strings  would  be  lifeless  and  unsatisfying  because  of 
the  absence  of  overtones. 

It  is  not  necessary  to  hold  finger  or  pencil  at  the  points 
1:3,  1:4,  etc.,  in  order  to  cause  the  string  to  vibrate  in  va- 
rious ways ;  if  a  string  is  merely  plucked  or  bowed  at  those 
places,  the  result  will  be  the  same.  It  is  important  to  re- 
member that  no  matter  where  a  string  of  definite  length  is 
bowed,  the  note  most  distinctly  heard  will  be  the  funda- 
mental; but  the  quality  of  the  emitted  tone  will  vary  with 
the  bowing.  For  example,  if  a  string  is  bowed  in  the  mid- 
dle, the  effect  will  be  far  less  pleasing  than  though  it  were 
bowed  near  the  end.  In  the  piano,  the  hammers  are  ar- 
ranged so  as  to  strike  near  one  end  of  the  string,  at  a  dis- 
tance of  about  I  :  7  to  1:9;  and  hence  a  large  number  of 
overtones  combine  to  reenforce  and  enrich  the  fundamental 
tone. 

270.  The  Individuality  of  Instruments.  It  has  been  shown 
that  a  piano  string  when  struck  by  a  hammer,  or  a  violin 
string  when  bowed,  or  a  mandolin  string  when  plucked, 
vibrates  not  only  as  a  whole,  but  also  in  segments,  and  as  a 
result  give  forth  not  a  simple  tone,  as  we  are  accustomed  to 
think,  but  a  very  complex  tone  consisting  of  the  fundamental 
and  one  or  more  overtones.  If  the  string  whose  fundamental 
note  is  lower  C  (128  vibrations  per  second)  is  thrown  into 
vibration,  the  tone  produced  may  contain,  in  addition  to  the 
prominent  fundamental,  any  one  or  more  of  the  following 
overtones :  C',  A",  C",E",C'",  etc. 


2Q2  MUSICAL   INSTRUMENTS 

The  number  of  overtones  actually  present  depends  upon  a 
variety  of  circumstances  :  in  the  piano,  it  depends  largely  upon 
the  location  of  the  hammer ;  in  the  violin,  upon  the  place  and 
manner  of  bowing.  Mechanical  differences  in  construction 
account  for  prominent  and  numerous  overtones  in  some  in- 
struments and  for  feeble  and  few  overtones  in  others.  The 
oboe,  for  example,  is  so  constructed  that  only  the  high  over- 
tones are  present,  and  hence  the  sound  gives  a  "  pungent " 
effect ;  the  clarinet  is  so  constructed  that  the  even-numbered 
overtones  are  killed,  and  the  presence  of  only  odd-numbered 
overtones  gives  individuality  to  the  instrument.  In  these  two 
instruments  we  have  vibrating  air  columns  instead  of  vibrat- 
ing strings,  but  the  laws  which  govern  vibrating  strings  are 
applicable  to  vibrating  columns  of  air,  as  we  shall  see  later. 
It  is  really  the  presence  or  absence  of  overtones  which  en- 
ables us  to  distinguish  the  note  of  the  piano  from  that  of  the 
violin,  flute,  or  clarinet.  If  overtones  could  be  eliminated, 
then  middle  C,  or  any  other  note  on  the  piano,  would  be  in- 
distinguishable from  that  same  note  sound  on  any  other  in- 
strument. The  fundamental  note  in  every  instrument  is  the 
same,  but  the  overtones  vary  with  the  instrument  and  lend  in- 
dividuality to  each.  The  presence  of  high  overtones  in  the 
oboe  and  the  presence  of  odd-numbered  overtones  in  the 
clarinet  enable  us  to  distinguish  without  fail  the  sounds  given 
out  by  these  instruments. 

The  richness  and  individuality  of  an  instrument  are  due, 
not  only  to  the  overtones  which  accompany  the  fundamental, 
but  also  to  the  "  forced  "  vibrations  of  the  inclosing  case,  or  of 
the  sounding  board.  If  a  vibrating  tuning  fork  is  held  in  the 
hand,  the  sound  will  be  inaudible  except  to  those  quite  near ; 
if,  however,  the  base  of  the  fork  is  held  against  the  table, 
the  sound  is  greatly  intensified  and  becomes  plainly  audible 
throughout  the  room. 


THE  KINDS  OF  STRINGED  INSTRUMENTS         293 

The  vibrations  of  the  fork  are  transmitted  to  the  table  top 
and  throw  it  into  vibrations  similar  to  its  own,  and  these  addi- 
tional vibrations  intensify  the  original  sound.  Any  fork,  no 
matter  what  its  frequency,  can  force  the  surface  of  the  table 
into  vibration,  and  hence  the  sound  of  any  fork  will  be  intensified 
by  contact  with  a  table  or  box. 

This  is  equally  true  of  strings;  if  stretched  between  two 
posts  and  bowed,  the  sound  given  out  by  a  string  is  feeble, 
but  if  stretched  over  a  sounding  board,  as  in  the  piano,  or 
over  a  wooden  shell,  as  in  the  violin,  the  sound  is  intensified. 
Any  note  of  the  instrument  will  force  the  sounding  body  to 
vibrate,  thus  reenforcing  the  volume  of  sound,  but  some  tones, 
or  modes  of  vibration,  do  this  more  easily  than  others,  and 
while  the  sounding  board  or  shell  always  responds,  it  re- 
sponds in  varying  degree.  Here  again  we  have  not  only  en- 
richment of  sound  but  also  individuality  of  instruments. 

271.  The  Kinds  of  Stringed  Instruments.  Stringed  instru- 
ments may  be  grouped  in  the  following  three  classes :  — 

a.  Instruments  in  which  the  strings  are  set  into  motion  by 

hammers  —  piano. 

b.  Instruments  in  which  the  strings  are  set  into    motion 

by  bowing  —  violin,  viola,  violoncello,  double  bass. 

c.  Instruments  in  which  the  strings  are  set  into  motion  by 

plucking  —  harp,  guitar,  mandolin. 

a.  The  piano  is  too  well  known  to  need  comment.  In 
passing,  it  may  be  mentioned  that  in  the  construc- 
tion of  the  modern  concert  piano  approximately 
40,000  separate  pieces  of  material  are  used.  The 
large  number  of  pieces  is  due,  partly,  to  the  fact 
that  the  single  string  corresponding  to  any  one  key 
is  usually  replaced  by  no. less  than  three  or  four 
similar  strings  in  order  that  greater  volume  of 
sound  may  be  obtained.  The  hammer  connected 


294 


MUSICAL  INSTRUMENTS 


FIG.  186.  —  i,  violin;  2,  viola;  3,  violoncello;  4,  double  bass. 


WIND  INSTRUMENTS 


295 


to  a  key  strikes  four  or  more  strings  instead  of  one, 
and  hence  produces  a  greater  richness  of  tone. 

b.  The  viola  is  larger  than  the  violin,  has  heavier  and 

thicker  strings,  and  is  pitched  to  a  lower  key ;  in 
all  other  respects  the  two  are  similar.  The  vio- 
loncello, because  of  the  length  and  thickness  of  its 
strings,  is  pitched  a  whole  octave  lower  than  the 
violin ;  otherwise  it  is  similar.  The  unusual 
length  and  thickness  of  the  strings  of  the  double 
bass  make  it  produce  very  low  notes,  so  that  it 
is  ordinarily  looked  upon 
as  the  "bass  voice"  of 
the  orchestra. 

c.  The  harp  has  always  been 

considered  one  of  the 
most  pleasing  and  per- 
fect of  musical  instru- 
ments. Here  the  skilled 
performer  has  absolutely 
free  scope  for  his  genius, 
because  his  fingers  can 
pluck  the  strings  at  will 
and  hence  regulate  the 
overtones,  and  his  feet 
can  regulate  at  will  the 
tension,  and  hence  the 
pitch  of  the  strings. 
Guitar  and  mandolin  are 
agreeable  instruments 
for  amateurs,  but  contain 
little  to  recommend  them  for  general  use. 
272.  Wind  Instruments.  In  the  so-called  wind  instruments, 
sound  is  produced  by  vibrating  columns  of  air  inclosed  in 


FIG.  187.  —  A  harp. 


2Q6 


MUSICAL  INSTRUMENTS 


tubes  or  pipes  of  different  lengths.  The  air  column  is  thrown 
into  vibration  either  directly,  by  blowing  across  a  narrow 
opening  at  one  end  of  a  pipe  as  in  the  case  of  the  whistle,  or 
indirectly,  by  exciting  vibrations  in  a  thin  strip  of  wood  or 

metal,  called  a  reed,  which  in  turn 
communicates  its  vibrations  to  the 
air  column  within. 

The  snorter  the  air  column,  the 
higher  the  pitch.  This  agrees  with 
the  law  of  vibrating  strings  which 
gives  high  pitches  for  short  lengths. 
The  pitch  of  the  sound  emitted 
by  a  column  of  air  vibrating  within 
a  pipe  varies  according  to  the  fol- 
lowing laws :  — 

1.  The     shorter    the    pipe,    the 
higher  the  pitch. 

2.  The  pitch    of  a  note  emitted 
by    an    open    pipe   is   one    octave 
higher  than  that  of  a  closed  pipe  of 
equal  length. 

3.  Air  columns    vibrate   in    seg- 
ments just   as   do    strings,  and  the 
tone   emitted   by   a   pipe    of  given 
length  is  complex,  consisting  of  the 

fundamental  and  one  or  more  overtones.  The  greater  the 
number  of  overtones  present,  the  richer  the  tone  produced. 

273.  How  the  Various  Pitches  are  Produced.  With  a  pipe 
of  given  length,  for  example,  the  clarinet,  (Fig.  189,  i)  dif- 
ferent pitches  are  obtained  by  blowing  hard  or  gently,  thus 
causing  increased  'or  decreased  vibration  of  the  reed  ;  also  by 
pressing  keys  which  open  holes  in  the  tube  and  thus  shorten 
or  lengthen  the  vibrating  air  column,  and  produce  a  rise  or 


FIG.  188.  —  Open  organ  pipes  of 
'         different  pitch. 


HOW  THE   VARIOUS  PITCHES  ARE  PRODUCED    297 


fall  in  pitch.     In  the  oboe  (Fig.  189,  2),  the  air  column  is  set 

into  motion  by  means  of  two  reeds  placed  in  the  mouthpiece 

of  the  tube.     The  flute  (Fig.  189,  3)  is  the  only  one  of  all  the 

instruments  in  which 

the  air  is  set  into  mo-     ^SSS^^ma^^S^gmi^ 

tion  by  direct  blowing 

from    the   mouth,    as 

is  done,  for  instance, 

when  we  blow  into  a 

bottle  or  tube. 

In  organ  pipes  air 

FlG.  189.  —  i,  clarinet;  2,  oboe;  3,  flute. 

is   blown   across    the 

sharp  edge  at  the  opening  of  a  narrow  tube ;  the  thin,  sharp 
edge  is  thrown  into  vibration,  and  these  vibrations  communi- 
cate themselves  to  the  column  of  air  within  the  organ  pipe. 

For  different 
pitches,  pipes  of 
different  lengths 
are  used :  for  very 
low  pitches  long, 
closed  pipes  are 
used;  for  very 
high  pitches 
short,  open  pipes 
are  used.  The 
mechanism  of  the 
organ  is  such  that 
pressing  a  key 
allows  the  air  to 


FlG.  190.  —  I,  horn  ;  2,  trumpet ;  3,  trombone. 


rush  into  the  communicating  pipe  and  a  sound  is  produced 
characteristic  of  the  length  of  the  pipe. 

In  the  brass  wind  instruments  such  as  horn,  trombone,  and 
trumpet,  the  lips  of   the  player  vibrate  and  excite  the  air 


298 


MUSICAL  INSTRUMENTS 


within.     Varying  pitches  are  obtained  largely  by   the  vary- 
ing wind  pressure  of  the  musician ;  if  he  breathes  fast,  the 


I  2          L3  3 

FIG.  191.  —  i,  kettledrum ;  2,  bass  drum ;  3,  cymbals. 

pitch  rises  ;  if  he  breathes  slowly,  the  pitch  falls.     All  of  these 
instruments,  however,  except  the  trombone  possess  a  few  valves 


FIG.  192.  —  The  seating  arrangement  of  the  Philadelphia  orchestra. 


THE  PERCUSSION  INSTRUMENTS  299 

which,  on  being  pressed,  vary  the  length  of  the  tube  and 
alter  the  pitch  accordingly.  In  the  trombone,  valves  are  re- 
placed by  a  section  which  slides  in  and  out  and  shortens  or 
lengthens  the  tube. 

274.  The  Percussion  Instruments.  The  percussion  instru- 
ments, including  kettledrums,  bass  drums,  and  cymbals,  are 
the  least  important  of  all  the  musical  instruments;  and  are 
usually  of  service  merely  in  adding  to  the  excitement  and  gen- 
eral effect  of  an  orchestra. 

In  orchestral  music  the  various  instruments  are  grouped 
somewhat  as  shown  in  Figure  192. 


CHAPTER   XXIX 

\ 

SPEAKING  AND  HEARING 

275.  Speech.  The  human  voice  is  the  most  perfect  of  musi- 
cal instruments.  Within  the  throat,  two  elastic  bands  are  at- 
tached to  the  windpipe  at  the  place  commonly  called  Adam's 
apple ;  these  flexible  bands  have  received  the  name  of  vocal 
cords,  since  by  their  vibration  all  speech  is  produced.  In 
ordinary  breathing,  the  cords  are  loose  and  are  separated  by 

a  wide  opening  through  which  air 
enters  and  leaves  the  lungs.  When 
we  wish  to  speak,  muscular  effort 
stretches  the  cords,  draws  them  closer 
together,  and  reduces  the  opening 
between  them  to  a  narrow  slit,  as  in 
the  case  of  the  organ  pipe.  If  air 
FIG.  193.— The  vibration  of  the  from  the  lungs  is  sent  through  the 
^Ch°±aPnr±Cee.StheS°Und  "arrow  slit,  the  vocal  cords  or  bands 
are  thrown  into  rapid  vibration  and 

produce  sound.  The  pitch  of  the  sound  depends  upon  the 
tension  of  the  stretched  membranes,  and  since  this  can  be 
altered  by  muscular  action,  the  voice  can  be  modulated  at 
will.  In  times  of  excitement,  when  the  muscles  of  the  body 
in  general  are  in  a  state  of  great  tension,  the  pitch  is  likely  to 
be  uncommonly  high. 

Women's  voices  are  higher  than  men's  because  the  vocal 
cords  are  shorter  and  finer ;  even  though  muscular  tension  is 
relaxed  and  the  cords  are  made  looser,  the  pitch  of  a  woman's 
voice  does  not  fall  so  low  as  that  of  a  man's  voice  since  his 

300 


THE  EAR  301 

cords  are  naturally  much  longer  and  coarser.  The  difference 
between  a  soprano  and  an  alto  voice  is  merely  one  of  length 
and  tension  of  the  vocal  cords. 

Successful  singing  is  possible  only  when  the  vocal  cords 
are  readily  flexible  and  when  the  singer  can  supply  a  steady, 
continuous  blast  of  air  through  the  slit  between  the  cords. 
The  hoarseness  which  frequently  accompanies  cold  in  the 
head  is  due  to  the  filling  up  of  the  slit  with  mucus,  because 
when  this  happens,  the  vocal  cords  cannot  vibrate  properly. 

The  sounds  produced  by  the  vocal  cords  are  transformed 
into  speech  by  the  help  of  the  tongue  and  lips  which  modify 
the  shape  of  the  mouth  cavity.  How  this  is  accomplished  is 
unknown  ;  many  animals  have  a  speaking  apparatus  similar 
to  our  own,  nevertheless  man  is  the  only  animal  able  to 
transform  sound  into  speech.  The  birds  use  their  vocal 
cords  to  beautiful  advantage  in  singing,  far  surpassing  us  in 
many  ways,  but  the  power  of  speech  is  lacking. 

276.  The  Ear.  The  pulses  created  in  the  air  by  a  sound- 
ing body  are  received  by  the  ear  and  transmitted  by  the 
auditory  nerve  to  the  brain,  where  they  produce  the  sensation 
of  sound.  The  ear  is  capable  of  marvelous  discrimination 
and  accuracy.  "  In  order  to  form  an  idea  of  the  extent  of 
this  power  imagine  an  auditor  in  a  large  music  hall  where  a 
full  band  and  chorus  are  performing.  Here,  there  are 
sounds  mingled  together  of  all  varieties  of  pitch,  loudness, 
and  quality  ;  stringed  instruments,  wood  instruments,  brass 
instruments,  and  voices  of  many  different  kinds.  And  in 
addition  to  these  there  may  be  all  sorts  of  accidental  and 
irregular  sounds  and  noises,  such  as  the  trampling  and  shuf- 
fling of  feet,  the  hum  of  voices,  the  rustle  of  dress,  the  creak- 
ing of  doors,  and  many  others.  Now  it  must  be  remembered 
that  the  only  means  the  ear  has  of  becoming  aware  of  these 
simultaneous  sounds  is  by  the  condensations  and  rarefac- 


302 


SPEAKING   AND   HEARING 


tions  which  reach  it  ;  and  yet  when  the  sound  wave  meets 
the  nerves,  the  nerves  single  out  each  individual  element, 
and  convey  to  the  mind  of  the  hearer,  not  only  the  tones  and 
notes  of  every  instrument  in  the  orchestra,  but  the  character 
of  every  accidental  noise  ;  and  almost  as  distinctly  as  if  each 
single  tone  or  noise  were  heard  alone."  —  POLE. 

277.  The  Structure  of  the  Ear.  The  external  portion  of 
the  ear  acts  as  a  funnel  for  catching  sound  waves  and  leading 
them  into  the  canal,  where  they  strike  upon  the  ear  drum, 


FIG.  194.— The  ear. 

or  tympanic  membrane,  and  throw  it  into  vibration.  Unless 
the  ear  drum  is  very  flexible  there  cannot  be  perfect  response 
to  the  sound  waves  which  fall  upon  it ;  for  this  reason,  the 
glands  of  the  canal  secrete  a  wax  which  moistens  the  mem- 
brane and  keeps  it  flexible.  Lying  directly  back  of  the 
tympanic  membrane  is  a  cavity  filled  with  air  which  enters 
by  the  Eustachian  tube  ;  from  the  throat  air  enters  the  Eusta- 
chian  tube,  moves  along  it,  and  passes  into  the  ear  cavity. 
The  dull  crackling  noise  noticed  in  the  ear  when  one  swallows 


THE  PHONOGRAPH  303 

is  due  to  the  entrance  and  exit  of  air  in  the  tube.  Several 
small  bones  stretch  across  the  upper  portion  of  the  cavity 
and  make  a  bridge,  so  to  speak,  from  the  ear  drum  to  the  far 
wall  of  the  cavity.  It  is  by  means  of  these  three  bones  that 
the  vibrations  of  the  ear  drum  are  transmitted  to  the  inner 
wall  of  the  cavity.  Behind  the  first  cavity  is  a  second  cavity 
so  complex  and  irregular  that  it  is  called  the  labyrinth  of  the 
ear.  This  labyrinth  is  filled  with  a  fluid  in  which  are  spread 
out  the  delicate  sensitive  fibers  of  the  auditory  nerves;  and  it 
is  to  these  that  the  vibrations  must  be  transmitted. 

Suppose  a  note  of  800  vibrations  per  second  is  sung.  Then 
800  pulses  of  air  will  reach  the  ear  each  second,  and  the  ear 
drum,  being  flexible,  will  respond  and  will  vibrate  at  the  same 
rate.  The  vibration  of  the  ear  drum  will  be  transmitted  by 
the  three  bones  and  the  fluid  to  the  fibers  of  the  auditory 
nerves.  The  tremors  imparted  to  the  auditory  nerve  reach 
the  brain  and  in  some  unknown  way  are  translated  into  sound. 

278.  Care  of  the  Ear.     Most  catarrhal  troubles  are  accom- 
panied by  an  oversupply  of  mucus  which  frequently  clogs  up  the 
Eustachian  tube  and  produces  deafness.     For  the  same  reason, 
colds  and  sore  throat  sometimes  induce  temporary  deafness. 

The  wax  of  the  ear  is  essential  for  flexibility  of  the  ear  drum  ; 
if  an  extra  amount  accumulates,  it  can  be  got  rid  of  by  bathing 
the  ear  in  hot  water,  since  the  heat  will  melt  the  wax.  The 
wax  should  never  be  picked  out  with  pin-or  sharp  object  except 
by  a  physician,  lest  injury  be  done  to  the  tympanic  membrane. 

279.  The  Phonograph.     The  invention  of  the  phonograph 
by  Edison  in   1878  marked  a  new  era  in  the  popularity  and 
dissemination  of  music.     Up  to  that  time,  household  music 
was  limited  to  those  who  were  rich  enough  to  possess  a  real 
musical  instrument,  and  who  in  addition  had  the  understand 
ing  and  the  skill  to  use  the  instrument.     The  invention  of  the 
phonograph  has  brought  music  to  thousands  of  homes  pos- 


304  SPEAKING  AND  HEARING 

sessed  of  neither  wealth  nor  skill.  That  the  music  reproduced 
by  a  phonograph  is  in  general  of  a  low  order,  vulgar  operas, 
coarse  songs,  etc.,  does  not  detract  from  the  interest  and  won- 
der of  the  instrument.  It  can  reproduce  what  it  is  called 
upon  to  reproduce,  and  if  human  nature  demands  vulgarity, 
the  instrument  will  be  made  to  satisfy  the  demand.  On  the 
other  hand,  speeches  of  famous  men,  national  songs,  magnifi- 
cent opera  selections,  and  other  pleasing  and  instructive  pro- 
ductions can  be  reproduced  fairly  accurately.  In  this  way 
the  phonograph,  perhaps  more  than  any  other  recent  inven- 
tion, can  carry  to  the  "  shut-ins  "  a  lively  glimpse  of  the  out- 
side world  and  its  doings. 

The  phonograph  consists  of  a  cylinder  or  disk  of  wax  upon 
which  the  vibrations  of  a  sensitive  metallic  disk  are  recorded 

by  means  of  a  fine 
metal  point.  The 
action  of  the  pointer  in 
reporting  the  vibra- 
tions of  a  disk  is  easily 
understood  by  refer- 
ence to  a  tuning  fork. 
Fasten  a  stiff  bristle 

FIG.  195. — A  vibrating  tuning  fork  traces  a  curved    £Q    a    tuning    fork     by 
line  on  smoked  glass. 

means  of  wax,  allow- 
ing the  end  of  the  point  to  rest  lightly  upon  a  piece  of  smoked 
glass.  If  the  glass  is  drawn  under  the  bristle  a  straight  line 
will  be  scratched  on  the  glass,  but  if  the  tuning  fork  is  struck 
so  that  the  prongs  vibrate  back  and  forth,  then  the  straight 
line  changes  to  a  wavy  line  and  the  type  of  wavy  line  depends 
upon  the  fork  used. 

In  the  phonograph,  a  disk  replaces  the  tuning  fork  and  a 
rotating  cylinder  coated  with  wax  replaces  the  glass  plate. 
When  the  speaker  talks  or  the  singer  sings,  his  voice  strikes 


THE  PHONOGRAPH 


305 


against  a  delicate  disk  and  throws  it  into  vibration,  and  the 
metal  point  attached  to  it  traces  on  the  wax  of  a  moving 
cylinder  a  groove  of  varying  shape  and  appearance  called 
the  "  record."  Every  variation  in  the  speaker's  voice  is 
repeated  in  the  vibrations  of  the  metal  disk  and  hence  in  the 
minute  motion  of  the  pointer  and  in  the  consequent  record 


FIG.  196.  —  A  phonograph. 

on  the  cylinder.  The  record  thus  made  can  be  placed  in  any 
other  phonograph  and  if  the  metal  pointer  of  this  new  phono- 
graph is  made  to  pass  over  the  tracing,  the  process  is  reversed 
and  the  Speaker's  voice  is  reproduced.  The  sound  given  out 
in  this-  way  is  faint  and  weak,  but  can  be  strengthened  by 
means  of  a  trumpet  attached  to  the  phonograph. 


CL.   GEN.    SCI.  —  20 


CHAPTER    XXX 

ELECTRICITY 

280.  Many  animals  possess  the  five  senses,  but  only  man 
possesses  constructive,  creative  power,  and  is  able  to  build  on 
the  information  gained  through  the  senses.  It  is  the  con- 
structive, creative  power  which  raises  man  above  the  level  of 
the  beast  and  enables  him  to  devise  and  fashion  wonderful 
inventions.  Among  the  most  important  of  his  inventions  are 
those  which  relate  to  electricity ;  inventions  such  as  trolley 
car,  elevator,  automobile,  electric  light,  the  telephone,  the 
telegraph.  Edison,  by  his  superior  construc- 
tive ability,  made  possible  the  practical  use  of 
the  telephone,  and  Marconi  that  of  wireless 
telegraphy.  To  these  inventions  might  be 
added  many  others  which  have  increased  the 
efficiency  and  production  of  the  business 
world  and  have  decreased  the  labor  and 
strain  of  domestic  life. 

281.  Electricity  as  first  Obtained  by  Man. 
Until  modern  times  the  only  electricity  known 
to  us  was  that  of  the  lightning  flash,  which 
man  could  neither  hinder  nor  make.  But 
in  the  year  1800,  electricity  in  the  form  of 
FIG.  197. -A  simple  a  weak  current  was  obtained  by  Volta  of 

electric  cell.  J 

Italy  in  a  very  simple  way ;  and  even  now 
our  various  electric  batteries  and  cells  are  but  a  modification 
of  that  used  by  Volta  and  called  a  voltaic  cell.  A  strip  of 

306 


EXPERIMENTS   WITH  THE   VOLTAIC  CELL         307 

copper  and  a  strip  of  zinc  are  placed  in  a  glass  containing 
dilute  sulphuric  acid,  a  solution  composed  of  oxygen,  hydro- 
gen, sulphur,  and  water.  As  soon  as  the  plates  are  immersed 
in  the  acid  solution,  minute  bubbles  of  gas  rise  from  the  zinc 
strip  and  it  begins  to  waste  away  slowly.  The  solution 
gradually  dissolves  the  zinc  and  at  the  same  time  gives  up 
some  of  the  hydrogen  which  it  contains ;  but  it  has  little  or 
no  effect  on  the  copper,  since  there  is  no  visible  change  in 
the  copper  strip. 

If,  now,  the  strips  are  connected  by  means  of  metal  wires, 
the  zinc  wastes  away  rapidly,  numerous  bubbles  of  hydrogen 
pass  over  to  the  copper  strip  and  collect  on  it,  and  a  current 
of  electricity  flows  through  the  connecting  wires.  Evidently, 
the  source  of  the  current  is  the  chemical  action  between  the 
zinc  and  the  liquid. 

Mere  inspection  of  the  connecting  wire  will  not  enable 
us  to  detect  that  a  current  is  flowing,  but  there  are  vari- 
ous ways  in  which  the  current  makes  itself  evident.  If  the 
ends  of  the  wires  attached  to  the  strips  are  brought  in  con- 
tact, a  faint  spark  passes,  and  if  the  ends  are  placed  on  the 
tongue,  a  twinge  is  felt. 

282.  Experiments  which  grew  out  of  the  Voltaic  Cell. 
Since  chemical  action  on  the  zinc  is  the  source  of  the  cur- 
rent, it  would  seem  reasonable  to  expect  a  current  if  the  cell 
consisted  of  two  zinc  plates  instead  of  one  zinc  plate  and 
one  copper  plate.  But  when  the  copper  strip  is  replaced  by 
a  zinc  strip  so  that  the  cell  consists  of  two  similar  plates,  no 
current  flows  between  them.  In  this  case,  chemical  action  is 
expended  in  heat  rather  than  in  the  production  of  electricity 
and  the  liquid  becomes  hot.  But  if  carbon  and  zinc  are 
used,  a  current  is  again  produced,  the  zinc  dissolving  away 
as  before,  and  bubbles  collecting  on  the  carbon  plate.  By 
experiment  it  has  been  found  that  many  different  metals  may 


308  ELECTRICITY 

be  employed  in  the  construction  of  an  electric  cell;  for  ex- 
ample, current  may  be  obtained  from  a  cell  made  with  a  zinc 
plate  and  a  platinum  plate,  or  from  a  cell  made  with  a  lead 
plate  and  a  copper  plate.  Then,  too,  some  other  chemical, 
such  as  bichromate  of  potassium,  or  ammonium  chloride,  may 
be  used  instead  of  dilute  sulphuric  acid. 

Almost  any  two  different  substances  will,  under  proper 
conditions,  give  a  current,  but  the  strength  of  the  current  is 
in  some  cases  so  weak  as  to  be  worthless  for  practical  use, 
such  as  telephoning,  or  ringing  a  door  bell.  What  is  wanted 
is  a  strong,  steady  current,  and  our  choice  of  material  is  limited 
to  the  substances  which  will  give  this  result.  Zinc  and  lead 
can  be  used,  but  the  current  resulting  is  weak  and  feeble,  and 
/  for  general  use  zinc  and  carbon  are  the  most  satisfactory. 

283.  Electrical  Terms.     The  plates  or  strips  used  in  mak- 
ing an  electric  cell  are  called  electrodes ;  the  zinc  is  called  the 
negative  electrode  ( —  ),  and  the  carbon   the    positive   elec- 
trode (  +  ) ;  the  current  is  considered  to  flow  through  the  wire 
from  the  -f-  to  the  —  electrode.     As  a  rule,  each  electrode 
has  attached  to  it  a  binding  post  to  which  wires  can  be 
quickly  fastened. 

The  power  that  causes  the  current  is  called  the  electro- 
motive force,  and  the  value  of  the  electromotive  force,  gen- 
erally written  E.  M.  F.,  of  a  cell  depends  upon  the  materials 
used. 

When  the  cell  consists  of  copper,  zinc,  and  dilute  sulphuric 
acid,  the  electromotive  force  has  a  definite  value  which  is 
always  the  same  no  matter  what  the  size  or  shape  of  the 
cell.  But  the  E.  M.  F.  has  a  decidedly  different  value  in  a 
cell  composed  of  iron,  copper,  and  chromic  acid.  Each  com- 
bination of  material  has  its  own  specific  electromotive  force. 

284.  The  Disadvantage  of  a  Simple  Cell.     When  the  poles 
of  a  simple  voltaic  cell  are  connected  by  a  wire,  the  current 


THE  GRAVITY  CELL 


309 


thus  produced  slowly  diminishes  in  strength  and,  after  a 
short  time,  becomes  feeble.  Examination  of  the  cell  shows 
that  the  copper  plate  is  covered  with  hydrogen  bubbles.  If 
these  bubbles  are  brushed  away  by  means  of  a  rod  or  stick, 
the  current  rises  to  its  former  strength,  but  as  the  bubbles 
again  gather  on  the  +  electrode  the  current  strength  di- 
minishes, and  when  the  bubbles  form  a  thick  film  on  the 
copper  plate,  the  current  is  too  weak  to  be  of  any  practical 
value.  The  film  of  bubbles  weakens  the  current  because  it 
practically  substitutes  a  hydrogen  plate  for  a  copper  plate, 
and  we  saw  in  Section  282  that  a  change  in  any  one  of 
the  materials  of  which  a  cell  is  composed  changes  the 
current. 

This  weakening  of  the  current  can  be  prevented  mechani- 
cally by  brushing  away  the  bubbles  as  soon  as  they  are  formed ; 
or  chemically,  by  surrounding  the  copper  plate  with  a  sub- 
stance which  will  combine  with  the  free  hydrogen  and  pre- 
vent it  from  passing  onward  to 
the  copper  plate. 

In  practically  all  cells,  the 
chemical  method  is  used  in  pref- 
erence to  the  mechanical  one. 
The  numerous  types  of  cells  in 
daily  use  differ  chiefly  in  the  de- 
vices employed  for  preventing 
the  formation  of  hydrogen  bub- 
bles, or  for  disposing  of  them  when 
formed.  One  of  the  best-known 
cells  in  which  weakening  of  the 
current  is  prevented  by  chemical 
means  is  the  so-called  gravity  cell. 

285.    The  Gravity   Cell.      A  large,  irregular  copper  elec- 
trode is  placed  in  the  bottom  of  a  jar  (Fig.  198),  and  com- 


FlG.  198.  —  The  gravity  cell. 


ELECTRICITY 


pletely  covered  with  a  saturated  solution  of  copper  sulphate. 
Then  a  large,  irregular  zinc  electrode  is  suspended  from  the 
top  of  the  jar,  and  is  completely  covered  with  dilute  sulphuric 
acid  which  does  not  mix  with  the  copper  sulphate,  but  floats 
on  the  top  of  it  like  oil  on  water.  The  hydrogen  formed  by 
the  chemical  action  of  the  dilute  sulphuric  acid  on  the  zinc 
moves  toward  the  copper  electrode,  as  in  the  simple  voltaic 
cell.  It  does  not  reach  the  electrode,  however,  because,  when 
it  comes  in  contact  with  the  copper  sulphate,  it  changes  places 
with  the  copper  there,  setting  it  free,  but  itself  entering  into 
the  solution.  The  copper  freed  from  the  copper  sulphate 
solution  travels  to  the  copper  electrode,  and  is  deposited 
on  it  in  a  clean,  smooth  layer.  Instead  of  a  deposit  of 
hydrogen  there  is  a  deposit  of  copper,  and  falling  off  in 
current  is  prevented. 

The  gravity  cell  is  cheap,  easy  to  construct,  and  of  constant 
strength,  and  is  in  almost  universal  use  in  telegraphic  work. 

Practically  all  small  railroad  sta- 
tions and  local  telegraph  offices 
use  these  cells. 

286.  Dry  Cells.  The  gravity 
cell,  while  cheap  and  effective,  is 
inconvenient  for  general  use,  owing 
to  the  fact  that  it  cannot  be  easily 
transported,  and  the  dry  cell  has 
largely  supplanted  all  others,  be- 
cause of  the  ease  with  which  it 
can  be  taken  from  place  to  place. 
This  cell  consists  of  a  zinc  cup, 
within  which  is  a  carbon  rod ;  the 
space  between  the  cup  and  rod  is  packed  with  a  moist  paste 
containing  certain  chemicals.  The  moist,  paste  takes  the 
place  of  the  liquids  used  in  other  cells. 


FIG.  199.  —  A  dry  cell. 


A  BATTERY  OF  CELLS 


287.  A  Battery  of  Cells.  The  electromotive  force  of  one 
cell  may  not  give  a  current  strong  enough  to  ring  a  door  bell 
or  to  operate  a  telephone. 
But  by  using  a  number  of 
cells,  called  a  battery,  the 
current  may  be  increased 
to  almost  any  desired 
strength.  If  three  cells  are 
arranged  as  in  Figure  200, 
so  that  the  copper  of  one 
cell  is  connected  with  the 

zinc  of  another  cell,  the  electromotive  force  of  the  battery  will 
be  three  times  as  great  as  the  E.  M.  F.  of  a  single  cell.  If 
four  cells  are  arranged  in  the  same  way,  the  E.  M.  F.  of  the 
battery  is  four  times  as  great  as  the  E.  M.  F.  of  a  single  cell; 
when  five  cells  are  combined,  the  resulting  E.  M.  F.  is  five 
times  as  great. 


FIG.  200.  —  A  battery  of  three  cells. 


CHAPTER   XXXI 

SOME   USES   OF  ELECTRICITY 

288.  Heat.  Any  one  who  handles  electric  wires  knows 
that  they  are  more  or  less  heated  by  the  currents  which  flow 
through  them.  If  three  cells  are  arranged  as  in  Figure  200 
and  the  connecting  wire  is  coarse,  the  heating  of  the  wire  is 
scarcely  noticeable;  but  if  a  shorter  wire  of  the  same  kind 
is  used,  the  heat  produced  is  slightly  greater;  and  if  the 
coarse  wire  is  replaced  by  a  short,  fine  wire,  the  heating  of 
the  wire  becomes  very  marked.  We  are  accustomed  to  say 
that  a  wire  offers  resistance  to  the  flow  of  a  current;  that 
is,  whenever  a  current  meets  resistance,  heat  is  produced 
in  much,  the  same  way  as  when  mechanical  motion  meets  an 
obstacle  and  spends  its  energy  in  friction.  The  flow  of  elec- 
tricity along  a  wire  can  be  compared  to  the  flow -of  water 
through  pipes :  a  small  pipe  offers  a  greater  resistance  to  the 
flow  of  water  than  a  large  pipe ;  less  water  can  be  forced 
through  a  small  pipe  than  through  a  large  pipe,  but  the  fric- 
tion of  the  water  against  the  sides  of  the  small  pipe  is  much 
greater  than  in  the  large  one. 

So  it  is  with  the  electric  current.  In  fine  wires  the  resist- 
ance to  the  current  is  large  and  the  energy  of  the  battery  is 
expended  in  heat  rather  than  in  current.  If  the  heat  thus 
produced  is  very  great,  serious  consequences  may  arise ;  for 
example,  the  contact  of  a  hot  wire  with  wall  paper  or  dry 
beams  may  cause  fire,  Insurance  companies  demand  that 
the  wires  used  in  wiring  a  building  for  electric  lights  be  of  a. 

312 


ELECTRIC  STOVES  313 

size  suitable  to  the  current  to  be  carried,  otherwise  they  will 
not  take  the  risk  of  insurance.  The  greater  the  current  to 
be  carried,  the  coarser  is  the  wire  required  for  safety. 

289.  Electric  Stoves.  It  is  often  desirable  to  utilize  the 
electric  current  for  the  production  of  heat.  For  example, 
trolley  cars  are  heated  by  coils  of  wire 
under  the  seats.  The  coils  offer  so 
much  resistance  to  the  passage  of  a 
strong  current  through  them  that  they 
become  heated  and  warm  the  cars. 

Some  modern  houses  are  so  built  that 
electricity  is  received  into  them  from  the 
great  plants  where  it  is  generated,  and 
by  merely  turning  a  switch  or  inserting  a  FlG-  »i.  -  An  electric  iron 

J  J  on  a  metal  stand. 

plug,  electricity  is  constantly  available. 

In    consequence,   many  practical    applications  of  electricity 

are  possible,  among  which  are  flatiron  and  toaster. 

Within  the  flatiron  (Fig.  201),  is  a  mass  of  fine  wire  coiled 
as  shown  in  Figure  202;  as  soon  as  the  iron  is  connected  with 
the  house  supply  of  electricity,  current  flows  through  the  fine 
wire  which  thus  becomes  strongly  heated  and  gives  off  heat 
to  the  iron.  The  iron,  'when  once 
heated,  retains  an  even  temperature 
as  long  as  the  current  flows,  and  the 
laundress  is,  in  consequence,  free  from 
FIG.  202. -The  fine  wires  are  the  disadvantages  of  a  slowly  cooling 

strongly  heated  by  the  cur-     jrori)    an(}    Of     frequent    Substitution    of 
rent    which    flows    through  . 

them.  a  warm  iron  for  a  cold  one.     Electric 

irons  are  particularly  valuable  in  sum- 
mer, because  they  eliminate  the  necessity  for  a  strong  fire,  and 
spare  the  housewife  intense  heat.  In  addition,  the  user  is  not 
confined  to  the  laundry,  but  is  free  to  seek  the  coolest  part  of 
the  house,  the  only  requisite  being  an  electrical  connection. 


SOME  USES  OF  ELECTRICITY 


FIG.     203.  —  Bread     can 
toasted  by  electricity. 


The  toaster  (Fig.  203)  is  another  useful  electrical  device, 
since  by  means  of  it  toast  may  be  made  on  a  dining  table  or 

at   a  bedside.      The  small  electrical 
stove,  shown  in  Figure  204,  is  similar 
in  principle  to  the  flatiron,  but  in  it 
the  heating  coil  is  arranged  as  shown 
in   Figure    205.      To    the    physician 
electric  stoves  are  valuable,  since  his 
instruments  can  be  sterilized  in  water 
heated  by  the  stove ;  and  that  without 
fuel  or  odor  of  gas. 
A  convenient  device  is  seen  in  the  heating  pad  (Fig.  206), 
a  substitute    of   a    hot   water   bag. 
Embedded  in  some  soft  thick  sub- 
stance   are  the  insulated  wires   in 
which  heat  is  to  be  developed,  and 
over  this  is  placed  a  covering  of  felt. 
290.    Electric  Lights.     In  the  in- 
candescent bulbs  (Fig.  207)  which 
commonly  illuminate  our  buildings, 

the  resistance  offered  to  the  electric  current  by  a  fine  hairlike 
wire  develops  enough  heat  to  make  the  wire 
a  glowing  mass  of  heat  and  light.  This  thin 
filament  is  inclosed  in  a  glass  bulb  from 
which  the  air  has  been  exhausted;  the  ab- 
sence of  air  prevents  the  filament  from 
burning  away,  and  it  merely  glows  and 
radiates  its  light  to  the  room. 

291.    Blasting..    Until  recently,  dynamit- 
ing was  attended  with  serious  danger,  owing 
to  the  fact  that    the    person   who    applied 
the  torch  to  the   fuse  could    not  make  a  safe  retreat  before 
the  explosion.     Now  a  fine  wire  is  inserted  in  the  fuse,  and 


FIG.  204.  —  An  electric  stove. 


FIG.  205. — The  heat- 
ing element  in  the 
electric  stove. 


CHEMICAL  EFFECTS 


315 


FIG.  206.  —  Ah   electric  pad   serves   the 
same  purpose  as  a  hot  water  bag. 


when  everything  is  in  readiness,    the  ends  of    the  wire  are 
attached  to  the  poles  of  a  distant  battery  and  the  heat   de- 
veloped in  the  wire  ignites  the 
fuse. 

292.  Welding  of  Metals. 
Metals  are  fused  and  welded  by 
the  use  of  the  electric  current. 
The  metal  pieces  which  are  to 
be  welded  are  pressed  together 
and  a  powerful  current  is  passed 
through    their    junction.      So 
great  is  the  heat  developed  that 
the  metals  melt  and  fuse,  and 
on  cooling  show  perfect  union. 

293.  Chemical  Effects.      The  Plating  of  Gold,  Silver,  and 

Other  Metals.  If  strips  of  lead  or  rods  of 
carbon  are  connected  to  the  terminals  of 
a  voltaic  cell,  as  in  Figure  208,  and  are 
then  dipped  into  a  solution  of  copper  sul- 
phate, the  strip  in  connection  with  the  neg- 
ative terminal  of  the  cell  soon  becomes  thinly 
plated  with  a  coating  of  copper.  If  a  solu- 
tion of  silver  nitrate  is  used  in  place  of  the 
copper  sulphate,  the  coating  formed  will  be 
of  silver  instead  of  copper.  So  long  as  the 
current  flows  and  there  is  any  metal  present 
in  the  solution,  the  coating  continues  to 
form  on  the  negative  electrode,  and  becomes 
thicker  and  thicker. 

The  process  by  which  metal  is  taken  out 
of  solution,  as  silver  out  of  silver  nitrate  and 
copper  out  of  copper  sulphate,  and  is  in  turn  deposited  as  a 
coating  on  another  substance,  is  called  electroplating.     An 


FIG.  207.  —  An  in- 
candescent electric 
bulb. 


SOME  USES   OF  ELECTRICITY 


electric  current  is  able  to  separate  a  liquid  into  its  various 
constituents  and  to  deposit  one  of  the  metal  constituents  on 

the  negative  electrode. 

Since  copper  is  constantly 
taken  out  of  the  solution  of  cop- 
per sulphate  for  deposit  upon  the 
negative  electrode,  the  amount  of 
copper  remaining  in  the  solution 
steadily  decreases,  and  finally  there 
is  none  of  it  left  for  deposit.  In 
order  to  overcome  this,  the  posi- 
FIG.  208, -Carbon  rods  in  a  solution  j  electrode  should  be  made  of 

of  copper  sulphate. 

the  same  metal  as  that  which  is 

to  be  deposited.  The  positive  metal  electrode  gradually  dis- 
solves and  replaces  the  metal  lost  from  the  solution  by  deposit 
and  electroplating  can  continue  as  long  as  any  positive 
electrode  remains. 

Practically  all  silver,  gold,  and 
nickel  plating  is  done  in  this  way ; 
machine,  bicycle,  and  motor  attach- 
ments are  not  solid,  but  are  of 
cheaper  material  electrically  plated 
with  nickel.  When  spoons  are  to 
be  plated,  they  are  hung  in  a  bath 
of  silver  nitrate  side  by  side  with 
a  thick  slab  of  pure  silver,  as  in 
Figure  209.  The  spoons  are  con- 
nected with  the  negative  terminal 
of  the  battery,  while  the  slab  of 
pure  silver  is  connected  with  the 
positive  terminal  of  the  same  bat- 
tery. The  length  of  time  that  the  current  flows  determines 
the  thickness  of  the  plating. 


FlG.  209.  —  Plating  spoons  by  elec- 
tricity. 


PRINTING  317 

294.  How  Pure  Metal  is  obtained  from  Ore.     When  ore  is 
mined,  it  contains  in  addition  to  the  desired  metal  many  other 
substances.     In  order  to  separate  out  the  desired  metal,  the 
ore  is  placed  in  some  suitable  acid  bath,  and  is  connected  with 
the  positive  terminal  of  a  battery,  thus  taking  the  place  of 
the  silver  slab  in  the  last   Section.      When  current  flows, 
any  pure  metal  which  is  present  is  dissolved  out  of  the  ore 
and  is  deposited  on  a  convenient  negative  electrode,  while 
the  impurities  remain  in  the  ore  or  drop  as  sediment  to  the 
bottom  of   the  vessel.      Metals  separated  from  the  ore  by 
electricity  are  called  electrolytic  metals  and   are  the  purest 
obtainable. 

295.  Printing.     The  ability  of  the  electric  current  to  de- 
compose a  liquid  and  to  deposit  a  metal  constituent  has  prac- 
tically revolutionized  the  process  of  printing.     Formerly,  type 
was  arranged  and  retained  in  position  until  the  required  num- 
ber of  impressions  had  been  made,  the  type  meanwhile  being 
unavailable  for  other  uses.     Moreover,  the  printing  of  a  sec- 
ond edition  necessitated  practically  as  great  labor  as  did  the 
first  edition,  the  type  being  necessarily  set  afresh.     Now,  how- 
ever, the  type  is  set  up  and  a  mold  of  it  is  taken  in  wax.     This 
mold  is  coated  with  graphite  to  make  it  a  conductor  and  is 
then  suspended  in  a  bath  of  copper  sulphate,  side  by  side 
with  a  slab  of   pure  copper.      Current  is  sent  through  the 
solution  as  described  in  Section  293,  until  a  thin  coating  of 
copper  has  been  deposited  on  the  mold.     The  mold  is  then 
taken  from  the  bath,  and  the  wax  is  replaced  by  some  metal 
which  gives  strength  and  support  to  the  thin  copper  plate. 
From  this  copper  plate,  which  is  an  exact  reproduction  of  the 
original  type,  many   thousand  copies  can  be  printed.      The 
plate  can  be  preserved  and  used  from  time  to  time  for  later 
editions,  and  the  original  type  can  be  put  back  into  the  cases 
and  used  again. 


CHAPTER   XXXII 


MODERN  ELECTRICAL  INVENTIONS 

296.  An  Electric  Current  acts  like  a  Magnet.  In  order  to 
understand  the  action  of  the  electric  bell,  we  must  consider  a 
third  effect  which  an  electric  current  can  cause.  Connect 
some  cells  as  shown  in  Figure  200  and  close  the  circuit 

through  a  stout  heavy  copper  wire, 
dipping  a  portion  of  the  wire  into 
fine  iron  filings.  A  thick  cluster 
of  filings  will  adhere  to  the  wire 
(Fig.  210),  and  will  continue  to 

as  t^e   current 


FIG.  210.  -A  wire  carrying  current  c\[ng  to  ft    SQ 
attracts  iron  filings. 

flows.     It  the  current  is  broken,  the 

filings  fall  from  the  wire,  and  only  so  long  as  the  current  flows 
through  the  wire  does  the  wire  have  power  to  attract  iron 
filings.  An  electric  current  transforms  a  wire  into  a  magnet, 
giving  it  the  power  to  attract  iron  filings. 

Although  such  a  straight  current  bearing  wire  attracts  iron 
filings,  its  power  of  attraction  is  very  small;  but  its  magnetic 
strength  can  be  in- 
creased by  coiling  as 
in  Figure  211.  Such 

_  FlG.  211.  —  A  loosely  wound  coil  of  wire. 

an  arrangement  of  wire 

is  known  as  a  helix  or  solenoid,  and  is  capable  of  lifting  or 
pulling  larger  and  more  numerous  filings  and  even  good- 
sized  pieces  of  iron,  such  as  tacks.  Filings  do  not  adhere 
to  the  sides  of  the  helix,  but  they  cling  in  clusters  to  the  ends 


THE  ELECTRIC  DELL  319 

of  the  coil.     This  shows  that  the  ends  of  the  helix  have  mag- 
netic power  but  not  the  sides. 

If  a  soft  iron  nail  (Fig.  212)  or  its  equivalent  is  slipped 
within  the  coil,  the  lifting  and  attractive  power  of  the  coil  is 
increased,  and  comparatively  heavy  weights  can  be  lifted. 

A  coil  of  wire  traversed  by  an  electric  current 
and  containing  a  core  of  soft  iron  has  the  power 
of  attracting  and  moving  heavy  iron  objects  ;  that  is, 
it  acts  like  a  magnet.  Such  an  arrangement  is 
called  an  electromagnet.  As  soon  as  the  current 
ceases  to  flow,  the  electromagnet  loses  its  power  and 
becomes  merely  iron  and  wire  without  magnetic 
properties. 

If  many  cells  are  used,  the  strength  of  the  electro- 
magnet is  increased,  and  if  the  coil  is  wound  closely,  FIG.  212.— 
as  in  Figure  213,  instead  of  loosely,  as  in  Figure     Coil  and 

soft   iron 

211,  the  magnetic  strength  is  still  further  increased,     rod. 
The    strength    of    any  electromagnet 
depends  upon  the  number  of  coils  wound  on 
the   iron  core  and  upon  the  strength  of  the 
current  which  is  sent  through  the  coils. 

To  increase  the  strength  of  the  electromagnet 
still  further,  the  so-called  horseshoe  shape  is 
used  (Fig.  214).  In  such  an  arrangement  there 
is  practically  the  strength  of  two  separate  elec- 
tromagnets. 

297.     The  Electric  Bell.     The  ringing  of  the 
electric  bell  is  due  to  the  attractive  power  of 
an  electromagnet.     By  the  pushing  of  a  button 
FIG.    213.  —  An  (Fig.  215)  connection  is  made  with  a  battery, 

electromagnet. 

and  current  nows  through  the  wire  wound  on 
the  iron  spools,  and  further  to  the  screw  P  which  presses 
against  the  soft  iron  strip  or  armature  S;  and  from  5  the 


320 


MODERN  ELECTRICAL  INVENTIONS 


current  flows  back  to  the  battery.  As  soon  as  the  current 
flows,  the  coils  become  magnetic  and  attract  the  soft  iron  arma- 
ture, drawing  it  forward  and  causing  the 
clapper  to  strike  the  bell.  In  this  position, 
vS  no  longer  touches  the  screw  /*,  and 
hence  there  is  no  complete  path  for  the 
electricity,  and  the  current  ceases.  But 
the  attractive,  magnetic  power  of  the 
coils  stops  as  soon  as  the  current  ceases; 
hence  there  is  nothing  to  hold  the  arma- 
ture down,  and  it  flies  back  to  its  former 
position. 

FIG.   214.  — A  horse-  T       j    • 

shoe  electromagnet  is  in& 

powerful     enough    to  this,tlOW- 
support  heavy  weights. 

ever,  the 

armature  makes  contact  at 
P  through  the  spring,  and 
the  current  flows  once  more; 
as  a  result  the  coils  again 
become  magnets,  the  ar- 
mature is  again  drawn  for- 
ward, and  the  clapper  again 
strikes  the  bell.  But  imme- 
diately afterwards  the  ar- 
mature springs  backward 
and  makes  contact  at  P 
and  the  entire  operation  is 
repeated.  So  long  as  we 
press  the  button  this  process 
continues  producing  what 
sounds  like  a  continuous 


FIG.  215.  —  The  electric  bell. 


jingle ;  in  reality  the  clapper  strikes  the  bell  every  time  a 
current  passes  through  the  electromagnet. 


HOW  ELECTRICITY  MAY  BE  LOST  TO    USE        321 

298.  The  Push  Button.     The  push  button  is  an  essential 
part  of  every  electric  bell,  because  without  it  the  bell  either 
would  not  ring  at  all,  or  would  ring  incessantly  until  the  cell 
was  exhausted.     When  the  push  button  is  free,  as  in  Fig- 
ure 216,  the  cell  terminals  are  not  connected  in  an  unbroken 
path,  and  hence  the  current  does 

not  flow.  When,  however,  the 
button  is  pressed,  the  current  has  a 
complete  path,  provided  there  is 

.  .  „  FIG.  216.  —  Push  button. 

the  proper  connection  at  5.     That 

is,  the  pressure  on  the  push  button  permits  current  to  flow 
to  the  bell.  The  flow  of  this  current  then  depends  solely 
upon  the  connection  at  S,  which  is  alternately  made  and 
broken,  and  in  this  way  produces  sound. 

The  sign  "Bell  out  of  order"  is  usually  due  to  the  fact  that 
the  battery  is  either  temporarily  or  permanently  exhausted. 
In  warm  weather  the  liquid  in  the  cell  may  dry  up  and  cause 
stoppage  of  the  current.  If  fresh  liquid  is  poured  into  the 
vessel  so  that  the  chemical  action  of  the  acid  on  the  zinc  is 
renewed,  the  current  again  flows.  Another  explanation  of 
an  out-of-order  bell  is  that  the  liquid  may  have  eaten  up  all 
the  zinc ;  if  this  is  the  case,  the  insertion  of  a  fresh  strip 
of  zinc  will  remove  the  difficulty  and  the  current  will  flow. 
If  dry  cells  are  used,  there  is  no  remedy  except  in  the  pur- 
chase of  new  cells. 

299.  How  Electricity  may  be  lost  to  Use.     In  the  electric 
bell,  we  saw  that  an  air  gap  at  the  push  button  stopped  the  flow 
of  electricity.     If  we  cut  the  wire  connecting  the  poles  of  a 
battery,   the   current   ceases   because  an  air  gap   intervenes 
and  electricity  does  not  readily  pass  through  air.     Many  sub- 
stances besides  air  stop  the  flow  of  electricity.     If  a  strip  of 
glass,  rubber,  mica,  or  paraffin  is  introduced  anywhere  in  a 
circuit,  the  current  ceases.     If  a  metal  is  inserted  in  the  gap, 

CL.   GEN.    SCI.  —  21 


322  MODERN  ELECTRICAL   INVENTIONS 

the  current  again  flows.  Substances  which,  like  an  air  gap, 
interfere  with  the  flow  of  electricity  are  called  non-conduc- 
tors, or,  more  commonly,  insulators.  Substances  which,  like 
the  earth,  the  human  body,  and  all  other  moist  objects,  con- 
duct electricity  are  conductors.  If  the  telephone  and  electric 
light  wires  in  our  houses  were  not  insulated  by  a  covering  of 
thread,  or  cloth,  or  other  non-conducting  material,  the  elec- 
tricity would  escape  into  surrounding  objects  instead  of  flow- 
ing through  the  wire  and  producing  sound  and  light. 

In  our  city  streets,  the  overhead  wires  are  supported'  on 
glass  knobs  or  are  closely  wrapped,  in  order  to  prevent  the 
escape  of  electricity  through  the  poles  to  the  ground.  In 
order  to  have  a  steady,  dependable  current,  the  wire  carry- 
ing the  current  must  be  insulated. 

Lack  of  insulation  means  not  only  the  loss  of  current  for 
practical  uses,  but  also  serious  consequences  in  the  event  of 
the  crossing  of  current-bearing  wires.  If  two  wires  properly 
insulated  touch  each  other,  the  currents  flow  along  their 
respective  wires  unaltered ;  if,  however,  two  uninsulated 
wires  touch,  some  of  the  electricity  flows  from  one  to  the 
other.  Heat  is  developed  as  a  result  of  this  transference, 
and  the  heat  thus  developed  is  sometimes  so  great  that  fire 
occurs.  For  this  reason,  wires  are  heavily  insulated  and 
extra  protection  is  provided  at  points  where  numerous  wires 
touch  or  cross. 

Conductors  and  insulators  are  necessary  to  the  efficient 
and  economic  flow  of  a  current,  the  insulator  preventing  the 
escape  of  electricity  and  lessening  the  danger  of  fire,  and 
the  conductor  carrying  the  current. 

300.  The  Telegraph.  Telegraphy  is  the  process  of  trans- 
mitting messages  from  place  to  place  by  means  of  an  electric 
current.  The  principle  underlying  the  action  of  the  tele- 
graph is  the  principle  upon  which  the  electric  bell  operates ; 


THE  TELEGRAPH  323 

namely,  that  a  piece  of  soft  iron  becomes  a  magnet  while  a 
current  flows  around  it,  but  loses  its  magnetism  as  soon  as     ,/ 
the  current  ceases. 

In  the  electric  bell,  the  electromagnet,  clapper,  push  button, 
and  battery  are  relatively  near,  —  usually  all  are  located  in 


FlG.  217. —  Diagram  of  the  electric  telegraph. 

the  same  building ;  while  in  the  telegraph  the  current  may 
travel  miles  before  it  reaches  the  electromagnet  and  produces 
motion  of  the  armature. 

The  fundamental  connections  of  the  telegraph  are  shown 
in  Figure  217.  If  the  key  K  is  pressed  down  by  an  operator 
in  Philadelphia,  the  current  from  the  battery  (only  one  cell 
is  shown  for  simplicity)  flows  through  the  line  to  New  York, 
passes  through  the  electromagnet  M,  and  thence  back  to  Phila- 
delphia. As  long  as  the  key  K  is  pressed  down,  the  coil  M 
acts  as  a  magnet  and  attracts  and  holds  fast  the  armature  A  ; 
but  as  soon  as  K  is  released,  the  current  is  broken,  M  loses 
its  magnetism,  and  the  armature  is  pulled  back  by  the  spring 
D.  By  a  mechanical  device,  tape  is  drawn  uniformly  under 
the  light  marker  P  attached  to  the  armature.  If  K  is  closed 
for  but  a  short  time,  the  armature  is  drawn  down  for  but 
a  short  interval,  and  the  marker  registers  a  dot  on  the  tape. 
If  K  is  closed  for  a  longer  time,  a  short  dash  is  made  by  the 
marker,  and,  in  general,  the  length  of  time  that  K  is  closed 
determines  the  length  of  the  marks  recorded  on  the  tape. 


324 


MODERN  ELECTRICAL   INVENTIONS 


The  telegraphic  alphabet  consists  of  dots  and  dashes  and 
their  various  combinations,  and  hence  an  interpretation  of 
the  dot  and  dash  symbols  recorded  on  the  tape  is  all  that  is 
necessary  for  the  receiving  of  a  telegraphic  message. 

The  Morse  telegraphic  code,  consisting  of  dots,  dashes,  and 
spaces,  is  given  in  Figure  218. 


A.. 

B_.. 

C.. 

]-)—•• 

E. 

F... 

G™ 


H.... 
I.. 
J  _____ 
"K_._ 
L  _ 
M__ 


O 
P 

Q 

lv 
S 
T 


U 
V 
W 
X 
Y 
Z 


FIG.  218.  —  The  Morse  telegraphic  code. 


The  telegraph  is  now  such  a  universal  means  of  communi- 
cation between  distant  points  that  one  wonders  how  business 
was  conducted  before  its  invention  in  1832  by  S.  F.  B.  Morse. 
301.  Improvements.  The  Sounder.  Shortly  after  the  in- 
vention of  telegraphy,  operators  learned  that  they  could  read 
the  message  by  the  click  of  the  marker  against  a  metal  rod 

which  took  the  place  of  the 
tape.  In  practically  all  tele- 
graph offices  of  the  present 
day  the  old-fashioned  tape 
is  replaced  by  the  sounder, 
shown  in  Figure  2  ig.  When 
current  flows,  a  lever,  L,  is 
drawn  down  by  the  electro- 
magnet and  strikes  against 
a  solid  metal  piece  with  a  click  ;  when  the  current  is  broken, 
the  lever  springs  upward,  strikes  another  metal  piece  and 
makes  a  different  click.  It  is  clear  that  the  working  of  the 
key  which  starts  and  stops  the  current  in  this  line  will  be  imi- 


FlG.  219.  —  The  sounder. 


IMPRO  VEMENTS 


325 


tated  by  the  motion  and  the  resulting  clicks  of  the  sounder. 
By  means  of  these  varying  clicks  of  the  sounder,  the  opera- 
tor interprets  the  message. 

The  Relay.  When  a  telegraph  line  is  very  long,  the  re- 
sistance of  the  wire  is  great,  and  the  current  which  passes 
through  the  electromagnet  is  correspondingly  weak,  so  feeble 
indeed  that  the  armature  must  be  made  very  thin  and  light 


FIG.  220. —  Diagram  of  a  modern  telegraph  system. 


in  order  to  be  affected  by  the  makes  and  breaks  in  the 
current.  The  clicks  of  an  armature  light  enough  to  respond 
to  the  weak  current  of  a  long  wire  are  too  faint  to  be  recogniz- 
able by  the  ear,  and  hence  in  such  long  circuits  some  device 
must  be  introduced  whereby  the  effect  is  increased.  This  is 
usually  done  by  installing  at  each  station  a  local  battery  and 
a  very  delicate  and  sensitive  electromagnet  called  the  relay. 
Under  these  conditions  the  current  of  the  main  line  is  not 
sent  through  the  sounder,  but  through  the  relay  which  opens 
and  closes  a  local  battery  in  connection  with  the  strong 


326  MODERN  ELECTRICAL  INVENTIONS 

sounder.  For  example,  the  relay  is  so  arranged  that  current 
from  the  main  line  runs  through  it  exactly  as  it  runs  through 
M  in  Figure  217.  When  current  is  made,  the  relay  attracts 
an  armature,  which  thereby  closes  a  circuit  in  a  local  battery 
and  thus  causes  a  click  of  the  sounder.  When  the  current 
in  the  main  line  is  broken,  the  relay  loses  its  magnetic  attrac- 
tion, its  armature  springs  back,  connection  is  broken  in  the 
local  circuit,  and  the  sounder  responds  by  allowing  its  arma- 
ture to  spring  back  with  a  sharp  sound. 

302.  The  Earth  an  Important  Part  of  a  Telegraphic  System. 
We  learned  in  Section  299  that  electricity  could  flow  through 
many  different  substances,  one  of  which  was  the  earth.     In 
all  ordinary  telegraph  and  telephone  lines,  advantage  is  taken 
of  this  fact  to  utilize  the  earth  as  a  conductor  and  to  dispense 
with  one  wire.  Originally  two  wires  were  used,  as  in  Figure  217; 
then  it  was  found  that  a  railroad  track  could  be  substituted 
for  one  wire,  and  later  that  the  earth  itself  served  equally 
well  for  a  return  wire.     The  present  arrangement  is  shown 
in  Figure  220,  where  there  is  but  one  wire,  the  circuit  being 
completed  by  the  earth.     No  fact  in  electricity  seems  more 
marvelous  than  that  the  thousands  of  messages  flashing  along 
the  wires  overhead  are  likewise  traveling  through  the  ground 
beneath.     If  it  were  not  for  this  use  of  the  earth  as  an  un- 
failing conductor,  the  network  of  overhead  wires  in  our  city 
streets  would  be  even  more  complex  than  it  now  is. 

303.  Advances  in  Telegraphy.     The  mechanical  improve- 
ments in  telegraphy  have  been  so  rapid   that  at  present  a 
single    operator   can    easily  send   or   receive    forty  words  a 
minute.     He  can  telegraph  more  quickly  than  the  average 
person  can  write ;   and  with  a  combination  of  the  latest  im- 
provements  the  speed  can  be  enormously   increased.     Re- 
cently, 1500  words  were  flashed  from  New  York  to  Boston 
over  a  single  wire  in  one  second.  , 


ADVANCES  IN  TELEGRAPHY  327 

In  actual  practice  messages  are  not  ordinarily  sent  long 
distances  over  a  direct  line,  but  are  automatically  transferred 
to  new  lines  at  definite  points.  For  example,  a  message  from 
New  York  to  Chicago  does  not  travel  along  an  uninterrupted 
path,  but  is  automatically  transferred  at  some  point,  such  as 
Lancaster,  to  a  second  line  which  carries  it  on  to  Pittsburgh, 
where  it  is  again  transferred  to  a  third  line  which  takes  it 
farther  on  to  its  destination. 


CHAPTER   XXXIII 


MAGNETS   AND   CURRENTS 

304.  In   the    twelfth   century,   there  was    introduced   into 
Europe  from  China  a  simple  instrument  which  changed  jour- 
neying on  the  sea  from  uncertain  wandering  to  a  definite,  safe 
voyage.     This  instrument  was  the  compass  (Fig.  221),  and  be- 
cause of  the  property  of  the  compass 
needle  (a  magnet)  to  point    unerr- 
ingly north  and  south,  sailors  were 
able  to  determine  directions  on  the 
sea   and   to    steer   for   the    desired 
point. 

Since  an  electric  current  is  prac- 
tically equivalent  to  a  magnet  (Sec- 
tion 296),  it  becomes  necessary  to 
know  the  most  important  facts 
relative  to  magnets,  facts  simple  in 
themselves  but  of  far-reaching 
value  and  consequences  in  electric- 
ity. Without  a  knowledge  of  the  magnetic  characteristics  of 
currents,  the  construction  of  the  motor  would  have  been 
impossible,  and  trolley  cars,  electric  fans,  motor  boats,  and 
other  equally  well-known  electrical  contrivances  would  be  un- 
known. 

305.  The  Attractive  Power  of  a  Magnet.     The  magnet  best 
known  to  us  all  is  the  compass  needle,  but  for  convenience 

328 


FiG.  221. —  The  compass. 


THE  EXTENT  OF  MAGNETIC  ATTRACTION 


329 


FIG.  222.  —  A  magnet. 


we  will  use  a  magnetic  needle  in  the  shape  of  a  bar  larger 
and  stronger  than  that  employed  in  the  compass.  If  we 
lay  such  a  magnet  on  a  pile  of  iron  filings,  it  will  be  found 
on  lifting  the  magnet  that  the  filings  cling  to  the  ends  in 
tufts,  but  leave  it  almost  bare  in  the  center  (Fig.  222).  The 
points  of  attraction  at 
the  two  ends  are  called 
the  poles  of  the  mag- 
net. 

If  a  delicately  made 
magnet  is  suspended 
as  in  Figure  223,  and 
is  allowed  to  swing 
freely,  it  will  always 
assume  a  definite  north 
and  south  position.  The  pole  which  points  north  when  the 
needle  is  suspended  is  called  the  north  pole  and  is  marked 
Nt  while  the  pole  which  points  south  when  the  needle  is 
suspended  is  called  the  south  pole  and  is  marked  S. 

A  freely  suspended  magnet  points  nearly  north  and  south. 
A  magnet    has  two  main  points  of  attraction 
called  respectively  the  north  and  south  poles. 

306.  The  Extent  of  Magnetic  Attraction.  If 
a  thin  sheet  of  paper  or  cardboard  is  laid  over  a 
strong,  bar-shaped  magnet  and  iron  filings  are 
FIG. 223. -The  then  gently  strewn  on  the  paper,  the  filings 
clearly  indicate  the  position  of  the  magnet  be- 
neath, and  if  the  cardboard  is  gently  tapped,  the 
filings  arrange  themselves  as  shown  in  Figure  224.  If  the 
paper  is  held  some  distance  above  the  magnet,  the  influence 
on  the  filings  is  less  definite,  and  finally,  if  the  paper  is  held 
very  far  away,  the  filings  do  not  respond  at  all,  but  lie  on 
the  cardboard  as  dropped. 


rn  H  £  n 
needle. 


330  MAGNETS  AND   CURRENTS 

The  magnetic  power  of  a  magnet,  while  not  confined  to 
the  magnet  itself,  does  not  extend  indefinitely  into  the  sur- 
rounding region ;  the  influence  is  strong  near  the  magnet, 
but  at  a  distance  becomes  so  weak  as  to  be  inappreciable. 
The  region  around  a  magnet  through  which  its  magnetic 


FlG.  224.  —  Iron  filings  scattered  over  a  magnet  arrange  themselves  in  definite  lines. 

force  is  felt  is  called  the  field  of  force,  or  simply  the  magnetic 
field,  and  the  definite  lines  in  which  the  filings  arrange  them- 
selves are  called  lines  of  force. 

The  magnetic  power  of  a  magnet  is  not  limited  to  the 
magnet,  but  extends  to  a  considerable  distance  in  all  direc- 
tions. 

307.  The  Influence  of  Magnets  upon  Each  Other.  If  while 
our  suspended  magnetic  needle  is  at  rest  in  its  characteristic 
north-and-south  direction  another  magnet  is  brought  near, 
the  suspended  magnet  is  turned ;  that  is,  motion  is  produced 
(Fig.  225).  If  the  north  pole  of  the  free  magnet  is  brought 
toward  the  south  pole  of  the  suspended  magnet,  the  latter 
moves  in  such  a  way  that  the  two  poles  N  and  5"  are  as  close 
together  as  possible.  If  the  north  pole  of  the  free  magnet 


ELECTRICITY  AS  A   MAGNET  331 

is  brought  toward  the  north  pole  of  the  suspended  magnet, 
the  latter  moves  in  such  a  way  that  the  two  poles  N  and  N 
are  as  far  apart  as  possible.  In  every  case  that  can  be 
tested,  it  is  found  that  a  north  pole  repels  a  north  pole,  and  a 
south  pole  repels  a  south 
pole;  but  that  a  north  and 
a  south  pole  always  attract 
each  other. 

The  main  facts  relative  to  /, 

magnets  may  be  summed  up  '  |_ 

as  follows  :  —  N 

a.  A    magnet     points 
nearly  north  and  south  if   it 

,,  j   ,  .         r  FlG.  225.  —  A  south  pole  attracts  a  north  pole. 

is  allowed  to  swing  freely. 

b.  A  magnet  contains  two  unlike  poles,  one  of  which  per- 
sistently points  north,  and  the  other  of  which  as  persistently 
points  south,  if  allowed  to  swing  freely. 

c.  Poles  of  the  same  name  repel  each  other ;  poles  of  un- 
like name  attract  each  other. 

d.  A  magnet  possesses  the  power  of  attracting  certain  sub- 
stances, like  iron,  and  this  power  of  attraction  is  not  limited 
to  the  magnet  itself  but  extends  into  the  region  around  the 
magnet. 

308.  Electricity  as  a  Magnet.  If  a  current  of  electricity 
transforms  a  wire  into  a  real  magnet,  such  a  wire  must  pos- 
sess all  of  the  characteristics  mentioned  in  the  preceding 
Section.  We  saw  in  Section  296  that  a  coiled  wire  through 
which  current  was  flowing  would  attract  iron  filings,  and  that 
the  attraction  was  practically  limited  to  the  two  ends  of  the 
helix.  That  a  coil  through  which  current  flows  possesses  the 
characteristics  a,  b,  c,  and  d,  of  a  magnet  is  shown  as  follows :  — 

a,  b.  If  a  helix  marked  at  one  end  with  a  red  string  is 
arranged  so  that  it  is  free  to  rotate  and  a  strong  current  is 


332 


MAGNETS  AND    CURRENTS 


FIG.  226.  —  A  helix  through  which  current 
flows  always  points  north  and  south,  if  it 
is  free  to  rotate. 


sent   through  it,  the   helix  will    immediately  turn  and   face 

about  until  it  points  north  and  south.     If  it  is  disturbed  from 

this  position,  it  will  slowly  swing  back  until  it  occupies  its 

characteristic  north  and  south 
position.  The  end  to  which 
the  string  is  attached  will  per- 
sistently point  either  north  or 
south.  If  the  current  is  sent 
through  the  coil  in  the  opposite 
direction,  the  two  poles  ex- 
change positions  and  the  helix 
turns  until  the  new  north  pole 
points  north. 

c.  If  a  coil  conducting  a 
current  is  held  near  a  sus- 
pended magnet,  one  end  of 

the  helix  will  be  found  to  attract  the  north  pole  of  the  magnet, 

while  the  opposite  end  will  be  found  to  repel  the  north  pole  of 

the  magnet.     In  fact,  the  helix  will  be  found  to  behave  in  every 

way  as   a  magnet,    with  a 

north  pole  at  one  end  and  a 

south  pole  at  the  other.     If 

the  current  is  sent  through 

the  helix  in  the  opposite  di- 

rection, the  north  and  south 

poles  exchange  places. 
If    the  number  of  turns 

in  the  helix  is  reduced  until 

hut  a    sinp-le    loon    remains 
m^ie  ns> 

the  result  is  the  same;  the 
single  loop  acts  like  a  flat  magnet,  one  side  of  the  loop  always 
facing  northward  and  one  southward,  and  one  face  attracting 
the  north  pole  of  the  suspended  magnet  and  one  repelling  it. 


FIG.  227.  -  A  wire  through  which  current  flows 
is  surrounded  by  a  field  of  magnetic  force. 


THE  PRINCIPLE  OF  THE  MOTOR 


333 


d.  If  a  wire  is  passed  through  a  card  and  a  strong  current 
is  sent  through  the  wire,  iron  filings  will,  when  sprinkled 
upon  the  card,  arrange  themselves  in  definite  directions 
(Fig.  227).  A  wire  carrying  a  current  is  surrounded  by  a 
magnetic  field  of  force. 

A  magnetic  needle  held  under  a  current-bearing  wire  turns 
on  its  pivot  and  finally  comes  to  rest  at  an  angle  with  the 
current.  The  fact  that  the  needle  is  deflected  by  the  wire 
shows  that  the  magnetic  power  of  the  wire  extends  into  the 
surrounding  medium. 

The  magnetic  properties  of  current  electricity  were  discov- 
ered by  Oersted  of  Denmark  less  than  a  hundred  years  ago ; 
but  since  that  time  practically  all  important  electrical  ma- 
chinery has  been  based  upon  one  or  more  of  the  magnetic 
properties  of  electricity.  The  motors  which  drive  our  electric 
fans,  our  mills,  and  our  trolley  cars  owe  their  existence  en- 
tirely to  the  magnetic  action  of  current 
electricity. 

309.  The  Principle  of  the  Motor.  If  a 
close  coil  of  wire  is  suspended  between  the 
poles  of  a  strong  horseshoe  magnet,  it  will 
not  assume  any  characteristic  position  but 
will  remain  wherever  placed.  If,  however, 
a  current  is  sent  through  the  wire,  the  coil 
faces  about  and  assumes  a  definite  position. 
This  is  because  a  coil,  carrying  a  current,  FIG.  228.  — The  coil 
is  equivalent  to  a  magnet  with  a  north  and 
south  face ;  and,  in  accordance  with  the 
magnetic  laws,  tends  to  move  until  its  north 
face  is  opposite  the  south  pole  of  the  horseshoe  magnet,  and 
its  south  face  opposite  the  north  pole  of  the  magnet.  If, 
when  the  coil  is  at  rest  in  this  position,  the  current  is  reversed, 
so  that  the  north  pole  of  the  coil  becomes  a  south  pole  and 


To  lattery 


turns  in  such  a  way 
that  its  north  pole  is 
opposite  the  south 
pole  of  the  magnet. 


334 


MAGNETS  AND   CURRENTS 


the  former  south  pole  becomes  a  north  pole,  the  result  is  that 
like  poles  of  coil  and  magnet  face  each  other.  But  since 
like  poles  repel  each  other,  the  coil  will  move,  and  will  rotate 
until  its  new  north  pole  is  opposite  to  the  south  pole  of  the 
magnet  and  its  new  south  pole  is  opposite  the  north  pole. 
By  sending  a  strong  current  through  the  coil,  the  helix  is 
made  to  rotate  through  a  half  turn  ;  by  reversing  the  current 
when  the  coil  is  at  the  half  turn,  the  helix  is  made  to  con- 
tinue its  rotation  and  to  swing  through  a  whole  turn.  If 
the  current  could  be  repeatedly  reversed  just  as  the  helix 
completed  its  half  turn,  the  motion  could  be  prolonged; 
periodic  current  reversal  would  produce  continuous  rotation. 
This  is  the  principle  of  the  motor. 

It  is  easy  to  see  that  long-continued  rotation  would  be  im- 
possible in  the  arrangement  of  Figure  228,  since  the  twisting 
of  the  suspending  wire  would  interfere  with  free  motion.  If 
the  motor  is  to  be  used  for  continuous  motion,  some  device 
must  be  employed  by  means  of  which  the  helix  is  capable  of 

continued    rotation 
around  its  support. 

In  practice,  the  rotat- 
ing coil  of  a  motor  is 
arranged  as  shown  in 
Figure  229.  Wires  from 
the  coil  terminate  on 
metal  disks  and  are  se- 
curely soldered  there. 
The  coil  and  disks  are 
supported  by  the  strong 

and  well-insulated  rod  R,  which  rests  upon  braces,  but  which 
nevertheless  rotates  freely  with  disks  and  coil.  The  current 
flows  to  the  coil  through  the  thin  metal  strips  called  brushes, 
which  rest  lightly  upon  the  disks. 


FIG.  229. —  Principle  of  the  motor. 


MECHANICAL   REVERSAL   OF  THE   CURRENT       335 


When  the  current  which  enters  at  B  flows  through  the 
wire,  the  coil  rotates,  tending  to  set  itself  so  that  its  north 
face  is  opposite  the  south  face  of  the  magnet.  If,  when  the 
helix  has  just  reached  this  position,  the  current  is  reversed  — 
entering  at  B'  instead  of  B — the  poles  of  the  coil  are  ex- 
changed ;  the  rotation,  therefore,  does  not  cease,  but  continues 
for  another  half  turn.  Proper  reversals  of  the  current  are 
accompanied  by  continuous  motion,  and  since  the  disk  and 
shaft  rotate  with  the  coil,  there  is  continuous  rotation. 

If  a  wheel  is  attached  to  the  rotating  shaft,  weights  can  be 
lifted,  and  if  a  belt  is  attached  to  the  wheel,  the  motion  of 
the  rotating  helix  can  be  transferred  to  machinery  for  prac- 
tical use. 

The  rotating  coil  is  usually  spoken  of  as  the  armature,  and 
the  large  magnet  as  the  field  magnet. 

310.  Mechanical  Reversal  of  the  Current.  The  Com- 
mutator. It  is  not  possible  by  hand  to  reverse  the  current 
with  sufficient  rapidity  and  precision  to  insure  uninterrupted 
rotation;  moreover,  the  physical  exertion  of  such  frequent 
reversals  is  considerable.  Hence,  some  mechanical  device 
for  periodically  reversing  the  current  is  necessary,  if  the 
motor  is  to  be  of  com- 
mercial value. 

The  mechanical  re- 
versal of  the  current  is 
accomplished  by  the 
use  of  the  commutator, 
which  is  a  metal  ring 
split  into  halves,  well 

insulated     from     each  FIG..**.- The  commutator, 

other     and    from    the 

shaft.     To  each  half  of  this  ring  is  attached  one  of  the  ends  of 
the  armature  wire.    The  brushes  which  carry  the  current  are  set 


336  MAGNETS  AND   CURRENTS 

on  opposite  sides  of  the  ring  and  do  not  rotate.  As  armature, 
commutator,  and  shaft  rotate,  the  brushes  connect  first  with 
one  segment  of  the  commutator  and  then  with  the  other. 
Since  the  circuit  is  arranged  so  that  the  current  always 
enters  the  commutator  through  the  brush  B,  the  flow  of  the 
current  into  the  coil  is  always  through  the  segment  in  contact 
with  B ;  but  the  segment  in  contact  with  B  changes  at  every 
half  turn  of  the  coil,  and  hence  the  direction  of  the  current 
through  the  coil  changes  periodically.  As  a  result  the  coil 
rotates  continuously,  and  produces  motion  so  long  as  current 
is  supplied  from  without. 

311.  The  Practical  Motor.  A  motor  constructed  in  accord- 
ance with  Section  309  would  be  of  little  value  in  practical 
everyday  affairs ;  its  armature  rotates  too  slowly  and  with 
too  little  force.  If  a  motor  is  to  be  of  real  service,  its  arma- 
ture must  rotate  with  sufficient  strength  to  impart  motion  to 
the  wheels  of  trolley  cars  and  mills,  to  drive  electric  fans,  and 
to  set  into  activity'  many  other  forms  of  machinery. 

The  strength  of  a  motor  may  be  increased  by  replacing  the 
singly  coiled  armature  by  one  closely  wound  on  an  iron 
core;  in  some  armatures  there  are  thousands  of  turns  of 
wire.  The  presence  of  soft  iron  within  the  armature  (Section 
296)  causes  greater  attraction  between  the  armature  and  the 
outside  magnet,  and  hence  greater  force  of  motion.  The 
magnetic  strength  of  the  field  magnet  influences  greatly  the 
speed  of  the  armature ;  the  stronger  the  field  magnet  the  greater 
the  motion,  so  electricians  make  every  effort  to  strengthen 
their  field  magnets.  The  strongest  known  magnets  are  elec- 
tromagnets, which,  as  we  have  seen,  are  merely  coils  of  wire 
wound  on  an  iron  core.  For  this  reason,  the  field  magnet  is 
usually  an  electromagnet. 

When  very  powerful  motors  are  necessary,  the  field  mag- 
net is  so  arranged  that  it  has  four  or  more  poles  instead  of 


THE  PRACTICAL  MOTOR 


337 


two ;  the  armature  likewise  consists  of  several  portions,  and 
even  the  commutator  may  be  very  complex.     But  no  matter 


FIG.  231.  —  A  modern  power  plant. 

how  compl'ex  these  various  parts  may  seem  to  be,  the  prin- 
ciple is  always  that  stated  in  Section  309,  and  the  parts  are 
limited  to  field  magnet,  commutator,  and  armature. 

The  motor  is  of  value  because  by  means  of  it  motion,  or 
mechanical  energy,  is  obtained  from  an  electric  current. 
Nearly  all  electric  street  cars  (Fig.  232),  are  set  in  motion  by 


Trolley  wire  or  Third  rail 


J- 

Track  — 


\  Motor 
FIG.  232.  —  The  electric  street  car. 

powerful  motors  placed  under  the  cars.     As   the  armature 
rotates,  its  motion  is  communicated  by  gears  to  the  wheels, 

CL.   GEN.    SCI.  —  22 


338  MAGNETS  AND   CURRENTS 

the  necessary  current  reaching  the  motor  through  the  over- 
head wires.  Small  motors  may  be  used  to  great  advantage 
in  the  home,  where  they  serve  to  turn  the  wheels  of  sew- 
ing machines,  and  to  operate  washing  machines.  Vacuum 
cleaners  are  frequently  run  by  motors. 


CHAPTER    XXXIV 

HOW   ELECTRICITY  MAY   BE   MEASURED 

312.  Danger  of  an  Oversupply  of  Current.  If  a  small  toy 
motor  is  connected  with  one  cell,  it  rotates  slowly;  if  con- 
nected with  two  cells,  it  rotates  more  rapidly,  and  in  general, 
the  greater  the  number  of  cells  used,  the  stronger  will  be  the 
action  of  the  motor.  But  it  is  possible  to  send  too  strong 
a  current  through  our  wire,  thereby  interfering  with  all  mo- 
tion and  destroying  the  motor.  We  have  seen  in  Section 
288  that  the  amount  of  current  which  can  safely  flow  through 
a  wire  depends  upon  the  thickness  of  the  wire.  A  strong 
current  sent  through  a  fine  wire  has  its  electrical  energy 
transformed  largely  into  heat ;  and  if  the  current  is  very 
strong,  the  heat  developed  may  be  sufficient  to  burn  off  the 
insulation  and  melt  the  wire  itself.  This  is  true  not  only  of 
motors,  but  of  all  electric  machinery  in  which  there  are  cur- 
rent-bearing wires.  The  current  should  not  be  greater  than 
the  wires  can  carry,  otherwise  too  much  heat  will  be  developed 
and  damage  will  be  done  to  instruments  and  surroundings. 

The  current  sent  through  our  electric  stoves  and  irons 
should  be  strong  enough  to  heat  the  coils,  but  not  strong 
enough  to  melt  them.  If  the  current  sent  through  our  electric 
light  wires  is  too  great  for  the  capacity  of  the  wires,  the  heat 
developed  will  injure  the  wires  and  may  cause  disastrous 
results.  The  overloading  of  wires  is  responsible  for  many 
disastrous  fires. 

339 


340  HOW  ELECTRICITY  MAY  BE  MEASURED 

The  danger  of  overloading  may  be  eliminated  by  inserting 
in  the  circuit  a  fuse  or  other  safety  device.  A  fuse  is  made 
by  combining  a  number  of  metals  in  such  a  way  that  the 
resulting  substance  has  a  low  melting  point  and  a  high  elec- 
trical resistance.  A  fuse  is  inserted  in  the  circuit,  and  the 
instant  the  current  increases  beyond  its  normal  amount  the 
fuse  melts,  breaks  the  circuit,  and  thus  protects  the  remain- 
ing part  of  the  circuit  from  the  danger  of  an  overload.  In 
this  way,  a  circuit  designed  to  carry  a  certain  current  is  pro- 
tected from  the  danger  of  an  accidental  overload.  The  noise 
made  by  the  burning  out  of  a  fuse  in  a  trolley  car  fre- 
quently alarms  passengers,  but  it  is  really  a  sign  that  the 
system  is  in  good  working  order  and  that  there  is  no  danger 
of  accident  from  too  strong  a  current. 

•  313.  How  Current  is  Measured.  The  preceding  Section  has 
shown  clearly  the  danger  of  too  strong  a  current,  and  the 
necessity  for  limiting  the  current  to  that  which  the  wire  can 
safely  carry.  There  are  times  when  it  is  desirable  to  know 
accurately  the  strength  of  a  current,  not  only  in  order  to 
guard  against  an  overload,  but  also  in  order  to  determine  in 
advance  the  mechanical  and  chemical  effects  which  will 
be  produced  by  the  current.  For  example,  the  strength  of 
the  current  determines  the  thickness  of  the  coating  of  silver 
which  forms  in  a  given  time  on  a  spoon  placed  in  an  electro- 
lytic bath ;  if  the  current  is  weak,  a  thin  plating  is  made  on 
the  spoon ;  if  the  current  is  strong,  a  thick  plating  is  made. 
If,  therefore,  the  exact  value  of  the  current  is  known,  the 
exact  amount  of  silver  which  will  be  deposited  on  the  spoon 
in  a  given  time  can  be  definitely  calculated. 

Current-measuring  instruments,  or  galvanometers,  depend 
for  their  action  on  the  magnetic  properties  of  current  elec- 
tricity. The  principle  of  practically  all  galvanometers  is  as 
follows:  — 


AMMETERS 


341 


FIG.    233.- 

The  prin- 
ciple of  the 
galvanome- 
ter. 


A  coil  of  wire  free  to  rotate  is  suspended  by  a  wire  be- 
tween the  poles  of  a  strong  magnet ;  when  a  current  is  sent 
through  the  coil,  the  coil  becomes  a  magnet  and  turns  so  that 
its  faces  will  be  towards  the  poles  of  the  perma- 
nent magnet.  But  as  the  coil  turns,  the  suspend- 
ing wire  becomes  twisted  and  hinders  the  turning. 
For  this  reason,  the  coil  can  turn  only  until  the 
motion  caused  by  the  current  is  balanced  by  the 
twist  of  the  suspending  wire.  But  the  stronger 
the  current  through  the  coil,  the  stronger  will  be 
the  force  tending  to  rotate  the  coil,  and  hence 
the  less  effective  will  be  the  hindrance  of  the 
twisting  string.  As  a  consequence,  the  coil 
swings  farther  than  before ;  that  is,  the  greater 
the  current,  the  farther  the  swing.  Usually  a 
delicate  pointer  is  attached  to  the  movable  coil 
and  rotates  freely  with  it,  so  that  the  swing 
of  the  pointer  indicates  the  relative  values  of  the  current. 
If  the  source  of  the  current  is  a  gravity  cell,  the  swing  of 
the  needle  is  only  two  thirds  as  great  as  when  a  dry  cell 
is  used,  indicating  that  the  dry  cell  furnishes  about  i  J  t;mes 
as  much  current  as  a  gravity  cell.1 

314.  Ammeters.  A  galvanometer  does  not  measure  the 
current,  but  merely  indicates  the  relative  strength  of  different 
currents.  But  it  is  desirable  at  times  to  measure  a  current 
in  units.  Instruments  for  measuring  the  strength  of  currents 
in  units  are  called  ammeters,  and  the  common  form  makes 
use  of  a  galvanometer. 

A  current  is  sent  through  a  movable  coil  (the  field  magnet 
and  coil  are  inclosed  in  the  case)  (Fig.  234),  and  the  magnetic 
field  thus  developed  causes  the  coil  to  turn,  and  the  pointer 

1  In  the  illustration,  soft  iron  has  been  placed  inside  the  coil  to  increase  the 

effect. 


342  HOW  ELECTRICITY  MAY  BE  MEASURED 

attached  to  it  to  move  over  a  scale  graduated  so  that  it  reads 
current  strengths.  This  scale  is  carefully  graduated  by  the 
following  method. 

If  two  silver  rods  (Fig.  209)  are  weighed  and  placed  in  a 
solution  of  silver  nitrate,  and  current  from  a  single  cell  is 
passed  through  the  liquid  for  a  definite  time,  we  find,  on 
weighing  the  two  rods,  that  one  has  gained  in  weight  and  the 
other  has  lost.  If  the  current  is  allowed  to  flow  twice  as  long, 
the  amount  of  silver  lost  and  gained  by  the  electrodes  is 
doubled ;  and  if  two  cells  are  used  instead  of  one,  the  result 
is  again  doubled. 

As  a  result  of  numerous  experiments,  it  was  found  that  a 
definite  current  of  electricity  will  deposit  a  definite  amount 
of  silver  in  a  definite  time,  and  that  the  amount  of  silver 
deposited  on  an  electrode  in  one  second  might  be  used  to 
measure  the  current  of  electricity  which  has  flowed  through 
the  circuit  in  one  second. 

A  current  is  said  to  be  one  ampere  strong  if  it  will  deposit 
silver  on  an  electrode  at  the  rate  of  .001 1 18  gram  per  second. 

In  marking  the  scale, 
an  ammeter  is  placed  in 
the  circuit  of  an  electro- 
lytic cell  and  the  position  of 
the  pointer  is  marked  on 
the  blank  card  which  lies 
beneath  and  which  is  to 
serve  as  a  scale  (Fig.  235). 
After  the  current  has 
flowed  for  about  an  hour, 
the  amount  of  silver  which 

FIG.  234.  —  An  ammeter.  .         .  .,  .  . 

has  been  deposited  is  meas- 
ured. Knowing  the  time  during  which  the  current  has 
run,  and  the  amount  of  deposit,  the  strength  of  the  cur- 


VOLTAGE  AND    VOLTMETERS  343 

rent  in  amperes  can  be  calculated.  This  number  is  written 
opposite  the  place  at  which  the  pointer  stood  during  the  ex- 
periment. 

The  scale  may  be  completed  by  marking  the  positions  of  the 
pointer  when  other  currents  of  known  strength  flow  through 
the  ammeter. 

All  electric  plants,  whether  for  heating,  lighting,  or  for 
machinery,  are  provided  with  ammeters,  such  instruments 


FIG.  235.  —  Marking  the  scale  of  an  ammeter. 

being  as  important  to  an  electric  plant  as  the  steam  gauge  is 
to  the  boiler. 

315.  Voltage  and  Voltmeters.  Since  electromotive  force,  or 
voltage,  is  the  cause  of  current,  it  should  be  possible  to  com- 
pare different  electromotive  forces  by  comparing  the  currents 
which  they  produce  in  a  given  circuit.  But  two  voltages  of 
equal  value  do  not  give  equal  currents  unless  the  resistances 
met  by  the  currents  are  equal.  For  example,  the  simple 
voltaic  cell  and  the  gravity  cell  have  approximately  equal 
voltages,  but  the  current  produced  by  the  voltaic  cell  is 
stronger  than  that  produced  by  the  gravity  cell.  This  is  be- 
cause the  current  meets  more  resistance  within  the  gravity 


344 


HO IV  ELECTRICITY  MAY  BE  MEASURED 


cell  than  within  the  voltaic  cell.  Every  cell,  no  matter  what 
its  nature,  offers  resistance  to  the  flow  of  electricity  through  it 
and  is  said  to  have  internal  resistance.  If  we  are  determin- 
ing the  voltages  of  various  cells  by  a  comparison  of  the 
respective  currents  produced,  the  result  will  be  true  only 
on  condition  that  the  resistances  in  the  various  circuits  are 
equal.  If  a  very  large  external  resistance  of  fine  wire  is 
placed  in  circuit  with  a  gravity  cell,  the  total  resistance  of  the 
circuit  (made  up  of  the  relatively  small  resistance  in  the  cell 
and  the  larger  resistance  in  the  rest  of  the  circuit)  will  differ 
but  little  from  that  of  another  circuit  in  which  the  gravity 
cell  is  replaced  by  a  voltaic  cell,  or  any  other  type  of  cell. 

With  a  high  resistance  in  the  outside  circuit,  the  deflections 
of  the  ammeter  will  be  small,  but  such  as  they  are,  they  will 
accurately  represent  the  electromotive  forces  which  produce 
them. 

Voltmeters  (Fig.  236),  or  instruments  for  measuring  volt- 
age, are  like  ammeters  except  that  a  wire  of  very  high 

resistance  is  in  circuit 
with  the  movable  coil.  In 
external  appearance  they 
are  not  distinguishable 
from  ammeters. 

The  unit  of  electromo- 
tive force  is  called  the  volt. 
The  voltage  of  a  dry  cell 
is  approximately  1.5  volts, 
and  the  voltage  of  a  voltaic 
cell  and  of  a  gravity  cell 
is  approximately  I  volt. 
316.    Current,  Voltage,  Resistance.     We  learned  in  Section 
287  that  the  strength  of  a  current  increases  when  the  electro- 
motive force  increases,  and  diminishes  when  the  electromotive 


FIG.  236.  — A  voltmeter. 


RESISTANCE  345 

force  diminishes.  Later,  in  Section  288,  we  learned  that  the 
strength  of  the  current  decreases  as  the  resistance  in  circuit 
increases. 

The  strength  of  a  steady  current  depends  upon  these  two 
factors  only,  the  electromotive  force  which  causes  it  and  the  ' 
resistance  which  it  has  to  overcome. 

317.    Resistance.     Since  resistance  plays  so  important  a  r61e 
in  electricity,  it  becomes  necessary  to  have  a  unit  of  resist- 
ance.    The  practical  unit  of  resistance  is  called  an  ohm,  and ' 
some  idea  of   the  value  of    an  ohm  can  be  obtained  if   we 
remember  that  a  3OO-foot  length  of  common  iron  telegraph 
wire  has  a  resistance  of  I  ohm.     An  approximate -ohm  for1 
rough  work  in  the  laboratory  may  be  made  by  winding  9  feet 
5  inches  of  number  30  copper  wire  on  a  spool  or  arranging 
it  in  any  other  convenient  form. 

In  Section  299  we  learned  that  substances  differ  very 
greatly  in  the  resistance  which  they  offer  to  electricity,  and 
so  it  will  not  surprise  us  to  learn  that  while  it  takes  300  feet 
of  iron  telegraph  wire  to  give  I  ohm  of  resistance,  it  takes 
but  39  feet  of  number  24  copper  wire,  and  but  2.2  feet  of 
number  24  German  silver  wire,  to  give  the  same  resistance. 

NOTE.  The  number  of  a  wire  indicates  its  diameter ;  number  30,  for 
example,  being  always  of  a  definite  fixed  diameter,  no  matter  what  the 
material  of  the  wire. 

If  we  wish  to  avoid  loss  of  current  by  heating,  we  use  a 
wire  of  low  resistance ;  while  if  we  wish  to  transform  elec- 
tricity into  heat,  as  in  the  electric  stove,  we  choose  wire  of 
high  resistance,  as  German  silver  wire. 


CHAPTER   XXXV 

HOW   ELECTRICITY   IS   MADE   ON   A   LARGE   SCALE 

318.  The  Dynamo.     We  have  learned  that  cells  furnish 
current  as  a  result  of  chemical  action,  and  that  the  substance 
usually  consumed  within  the  cell  is  zinc.     Just  as  coal  within 
the  furnace  furnishes  heat,  so  zinc  within  the  cell  furnishes 
electricity.     But  zinc  is  a  much  more  expensive  fuel  than 
coal  or  oil  or  gas,  and  to  run  a  large  motor  by  electricity  pro- 
duced in  this  way  would  be  very  much  more  expensive  than 
to  run  the  motor  by  water  or  steam.     For  weak  and  infre- 
quent currents  such  as  are  used  in  the  electric  bell,  only  small 
quantities  of  zinc  are  needed,  and  the  expense  is  small.     But 
for  the  production  of  such  powerful  currents  as  are  needed 
to  drive  trolley  cars,  elevators,  and  huge  machinery,  enormous 
quantities  of  zinc  would  be  necessary  and  the  cost  would  be 
prohibitive.     It  is  safe  to  say  that  electricity  would  never 
have  been  used  on  a  large  scale  if  some  less  expensive  and 
more  convenient  source  than  zinc  had  not  been  found. 

319.  A  New  Source  of  Electricity.     It  came  to  most  of  us 
as  a  surprise  that  an  electric  current  has  magnetic  properties 
and  transforms  a  coil  into  a  veritable  magnet.      Perhaps  it 
will  not  surprise  us  now  to  learn  that  a  magnet  in  motion 
has  electric  properties  and  is,  in  fact,  able  to  produce  a  current 
within  a  wire.     This  can  be  proved  as  follows:  — 

Attach  a  closely   wound  coil  to  a  sensitive  galvanometer 
(Fig.  237);  naturally  there  is  no  deflection  of  the  galvanom- 

346 


A   NEW  SOURCE  OF  ELECTRICITY 


347 


eter  needle,  because  there  is  no  current  in  the  wire.  Now 
thrust  a  magnet  into  the  coil.  Immediately  there  is  a  deflec- 
tion of  the  needle,  which  indicates  that  a  current  is  flow- 
ing through  the  circuit.  If  the  magnet  is  allowed  to  remain 
at  rest  within  the  coil,  the  needle  returns  to  its  zero  posi- 
tion, showing  that  the  current  had  ceased.  Now  let  the 


FIG.  237. — The  motion  of  a  magnet  within  a  coil  of  wire  produces  a  current  of  electricity. 

magnet  be  withdrawn  from  the  coil ;  the  needle  is  deflected 
as  before,  but  the  deflection  is  in  the  opposite  direction, 
showing  that  a  current  exists,  but  that  it  flows  in  the  opposite 
direction.  We  learn,  therefore,  that  a  current  may  be  in- 
duced in  a  coil  by  moving  a  magnet  back  and  forth  within 
the  coil,  but  that  a  magnet  at  rest  within  the  coil  has  no  such 
influence. 

An  electric  current  transforms  a  coil  into  a  magnet.  A 
magnet  in  motion  induces  electricity  within  a  coil ;  that  is, 
causes  a  current  to  flow  through  the  coil. 


348     HOW  ELECTRICITY  IS  MADE  ON  A  LARGE  SCALE 

A  magnet  possesses  lines  of  force,  and  as  the  magnet 
moves  toward  the  coil  it  carries  lines  of  force  with  it,  and  the 
coil  is  cut,  so  to  speak,  by  these  lines  of  force.  As  the  mag- 
net recedes  from  the  coil,  it  carries  lines  of  force  away  with 
it,  this  time  reducing  the  number  of  the  lines  which  cut  the 
coil. 

320.  A  Test  of  the  Preceding  Statement.     We  will  test  the 
statement  that  a  magnet  has  electric  properties  by  another  ex- 
periment.    Between  the  poles  of  a  strong  magnet 
suspend  a  movable  coil  which  is  connected  with 
a  sensitive  galvanometer   (Fig.    237).     Starting 
with  the  coil  in  the  position  of  Figure  228,  when 

r-  /— |  many  lines  of  force  pass  through  it,  let  the 
coil  be  rotated  quickly  until  it  reaches  the  posi- 
tion indicated  in  Figure  238,  when  no  lines  of 
force  pass  through  it.  During  the  motion  of  the 
coil,  a  strong  deflection  of  the  galvanometer  is 
observed  ;  but  the  deflection  ceases  as  soon  as  the 

FIG.    238.  —  As  ... 

long   as    the  coil  ceases,  to  rotate.     It,  now,  starting  with  the 
coil  rotates  be-  pOSitiOn  of  Figure  238,  the  coil  is  rotated  forward 

tween      the  v 

poles    of  the  to  its  starting   point,   a  deflection  occurs  in  the 
renf  flows  CUr~  °PP°site  direction,  showing  that  a  current  is  pres- 
ent, but  that  it  flows  in  the  opposite  direction. 
So  long  as  the  coil  is  in    motion,  it  is    cut    by    a    varying 
number  of  lines  of  force,  and  current  is  induced  in  the  coil. 
The   above   arrangement   is  a  dynamo  in   miniature.     By 
rotation  of   a  coil  (armature)  within  a  magnetic  field,  that  is, 
between  the  poles  of  a  magnet,  current  is  obtained. 

In  the  motor,  current  produces  motion.  In  the  dynamo, 
motion  produces  current. 

321.  The  Dynamo.     As  has  been  said,  the  arrangement  of 
the   preceding  Section    is    a   dynamo   in    miniature.     Every 
dynamo,  no  matter  how   complex  its  structure  and  appear- 


THE  DYNAMO  349 

ance,  consists  of  a  coil  of  wire  which  can  rotate  continuously 
between  the  poles  of  a  strong  magnet.  The  mechanical 
devices  to  insure  easy  rotation  are  similar  in  all  respects  to 
those  previously  described  for  the  motor. 

The  current  obtained  from    such  a  dynamo  alternates  in 
direction,  flowing  first  in  one  direction  and  then  in  the  oppo- 


FlG.  239.  —  A  modern  electrical  machine. 

site  direction.  Such  alternating  currents  are  unsatisfactory 
for  many  purposes,  and  to  be  of  service  are  in  many  cases 
transformed  into  direct  currents ;  that  is,  current  which  flows 
steadily  in  one  direction.  This  is  accomplished  by  the  use  of 
a  commutator.  In  the  construction  of  the  motor,  continuous 
motion  in  one  direction  is  obtained  by  the  use  of  a  commu- 
tator (Section  310);  in  the  construction  of  a  dynamo,  continu- 
ous current  in  one  direction  is  obtained  by  the  use  of  a  similar 
device. 


350     HOW  ELECTRICITY  IS  MADE  ON  A  LARGE  SCALE 

322.  Powerful  Dynamos.  The  power  and  efficiency  of  a 
dynamo  are  increased  by  employing  the  devices  previously 
mentioned  in  connection  with  the  motor.  Electromagnets 


FIG.  240.  —  Thomas  Edison,  one  of  the  foremost  electrical  inventors  of  the  present  day. 

are  used  in  place  of  simple  magnets,  and  the  armature, 
instead  of  being  a  simple  coil,  may  be  made  up  of  many  coils 
wound  on  soft  iron.  The  speed  with  which  the  armature  is 
rotated  influences  the  strength  of  the  induced  current,  and 
hence  the  armature  is  run  at  high  speed. 

The  average  dynamo,  such  as  is  used  for  lighting  fifty 
incandescent  lamps,  has  a  horse  power  of  about  33.5,  and 
large  dynamos  are  frequently  as  powerful  as  7500  horse 
power. 

323.  The  Telephone.  When  a  magnet  is  at  rest  within  a 
closed  coil  of  wire,  as  in  Section  319,  current  does  not  flow 
through  the  wire.  But  if  a  piece  of  iron  is  brought  near  the 


COST  OF  ELECTRIC  POWER  351 

magnet,  current  is  induced  and  flows  through  the  wire;  if  the 
iron  is  withdrawn,  current  is  again  induced  in  the  wire  but 
flows  in  the  opposite  direction.  As  iron  approaches  and 
recedes  from  the  magnet,  current  is  induced  in  the  wire 
surrounding  the  magnet.  This  is  in  brief  the  principle  of  the 
telephone.  When  one  talks  into  a  receiver,  Z,  the  voice 


FlG.  241. —  Diagram  of  a  simple  telephone  circuit. 

throws  into  vibration  a  sensitive  iron  plate  standing  before  an 
electromagnet.  The  back  and  forth  motion  of  the  iron  plate 
induces  current  in  the  electromagnet  c.  The  current  thus 
induced  makes  itself  evident  at  the  opposite  end  of  the  line 
M,  where  by  its  magnetic  attraction,  it  throws  a  second  iron 
plate  into  vibrations.  The  vibrations  of  the  second  plate 
are  similar  to  those  produced  in  the  first  plate  by  the  voice. 
The  vibrations  of  the  far  plate  thus  reproduce  the  sounds 
uttered  at  the  opposite  end. 

324.  Cost  of  Electric  Power.  The  water  power  of  a  stream 
depends  upon  the  quantity  of  water  and  the  force  with  which 
it  flows.  The  electric  power  of  a  current  depends  upon  the 
quantity  of  electricity  and  the  force  under  which  it  flows.  The 
unit  of  electric  power  is  called  the  .watt;  it  is  the  power  fur- 
nished by  a  current  of  one  ampere  with  a  voltage  of  one  volt. 

One  watt  represents  a  very  small  amount  of  electric  power, 
and  for  practical  purposes  a  unit  1000  times  as  large  is  used, 
namely,  the  kilowatt.  By  experiment  it  has  been  found  that 
one  kilowatt  is  equivalent  to  about  i^  horse  power.  Electric 
current  is  charged  for  by  the  watt  hour.  A  current  of  one 


352     HOW  ELECTRICITY  IS  MADE  OAT  A  LARGE  SCALE 

ampere,,  having  a  voltage  of  one  volt,  will  furnish  in  the 
course  of  one  hour  one  watt  hour  of  energy.  Energy  for 
electric  lighting  is  sold  at  the  rate  of  about  ten  cents  per 
kilowatt  hour.  For  other  purposes  it  is  less  expensive.  The 
meters  commonly  used  measure  the  amperes,  volts,  and  time 
automatically,  and  register  the  electric  power  supplied  in 
watt  hours. 


INDEX 


Absorption,   of  heat  by    lampblack, 
143-144. 

of  gases  by  charcoal,  57. 

of  light  waves,  135-138. 
Accommodation  of  the  eye,  123. 
Acetanilid,  259. 
Acetylene,  as  illuminant,  152-153. 

manufacture  of,  152-153. 

properties  of,   220. 
Acid,  boric,  253. 

carbolic,  152,  251,  252. 

hydrochloric,  55,  80,  227,  238,  241. 

lactic,  230. 

oxalic,  247,  248. 

salicylic,  253. 

sulphuric,  55,  80,  240,  241,  307. 

Sulphurous,  242. 
Acids,  action  on  litmus,  220. 
Adenoids,  51. 

Adulterants,  detection  of,  16. 
Air,  characteristics  of,  81-83,  86,  189. 

compressibility  of,  91. 

expansion  of,  10-11. 

humidity,  38,  39. 

pumps,  201-205. 

transmits  sound,  269. 

weight  of,  86. 

See  Atmosphere. 
Alcohol,  234. 

in  patent  medicines,  260. 
Alizarin,  248. 
Alkali,  222. 

Alternating  current,  351. 
Alum,  247. 

in  baking  powder,  230. 
Ammeter,  341,  343. 
Ammonia,  152. 

a  base,  221-222. 

in  bath,  226. 

in  manufacture  of  ice,  98. 

neutralizing  chlorine,  240. 
Ampere,  342. 

CL.  GEN.  SCI.  —  2  A 


Anemia,  259. 

Angle,  of  incidence,  110. 

of  reflection,  110. 

of  refraction,  114. 
Aniline,  152,  245. 
Animal  charcoal,  58. 
Animal  transportation,  132. 
Antichlor,  240. 
Antipyrin,  259. 
Armature,  319,  320. 

dynamo,  350. 

motor,  335. 

Artificial  lighting,  148-153. 
Atmosphere,  81. 

carbon  dioxide  in,  54-55. 

height  of,  81. 

nitrogen  and  oxygen  in,  262. 

pressure  of,  82-86. 

water  vapor  in,  36-38. 

weight,  86. 

See  Air. 

Atmospheric  pressure,  82-86 
Atomizer,  92. 
Atoms,  102. 

Automobiles,  gas  engines,  185. 
Axis  of  a  lens,  119. 

Bacteria,  133. 

as  nitrogen  makers,  263. 

destroyed  by  sunlight,  etc.,  133, 250, 
251. 

diseases  caused  by,  133. 

in  butter  and  cheese,  133 
Baking  powder,  229-230. 
Baking  soda,  227-229. 
Barograph,  87. 
Barometer,  aneroid,  84-85. 

mercury,  84. 

use  in  weather  predictions,  86-87. 
Bases,  action  on  litmus,  221-222. 

properties,  220-222. 
Battery,  electric,  311. 


353 


354 


INDEX 


Beans,  as  food,  66. 

roots  take  in  nitrogen,  263. 
Bell,  electric,  319-321. 
Benzine,  150. 

as  a  cleaning  agent,  227. 
Benzoate  of  soda,  253. 
Bicarbonate   of   soda,    in   fire   extin- 
guisher; 55,  56. 

in  Rochelle  salt,  227. 

in  soda  mints,  231. 

in  seidlitz  powder,  231. 
Bicycle  pumps,  202. 
Blasting,  by  electricity,  314. 
Bleaching,  237-243. 

by  chlorine,  238-240. 
Bleaching  powder,  239-240. 
Body,  human,  63-64. 

a  conductor  of  electricity,  322. 
Boiling,  31. 

amount  of  heat  absorbed,  31-32. 

of  milk,  32. 

of  water,  77. 
•    point,  15. 
Bomb  calorimeter,  61. 
Borax,  as  meat  preservative,  253. 

as  washing  powder,  226. 
Boric    acid,    as    meat    preservative, 

253. 

Boyle's  law,  95-96. 
Bread,  232-233. 

unleavened,  233. 
Bread  making,  232-235. 
Breathing,  hygienic  habits  of,  50. 

by  mouth,  50-51. 
Burns,  treatment  of,  52-53. 
Butter,  adulteration  test,  16. 

bacteria  in,  133. 
Buttermilk,  230. 

Caisson,  203-204. 
Calcium  carbide,  152-153. 

in    making    nitrogenous    fertilizer, 

264. 

Calico  printing,  249. 
Calorie,  27-28,  61-62. 
Calorimeter,  61. 
Camera,  128-129. 

films,  129. 

lens,  129. 

plates,  129. 

Camping,  water  supply,  195-197. 
Candle,  148-149. 


as  standard  for  light  measure,  104- 

105. 

Candle  power,  105-107. 
Carbide,  calcium,  152-153,  264. 
Carbohydrates,  64-65,  149. 
Carbolic  acid,  152. 

as  disinfectant,  251. 
Carbon,  56,  66. 

in  voltaic  cells,  308. 
Carbon  dioxide,  53. 

as  fire  extinguisher,  55-56. 

commercial  use,  55-56. 

in  baking  soda,  228. 

in  fermentation,  234. 

in  health,  54. 

in  plants,  55. 

preparation  of,  55. 

source  of,  53. 

test  for,  228. 
Catarrh,  259. 
Caustic  lime,  222. 
Caustic  potash,  222. 
Caustic  soda,  218,  222. 

to  make  a  salt,  227. 
Caves  and  caverns,  71. 
Cell,  dry,  310. 

gravity,  309-310. 

voltaic,  306-308,  310. 
Cells  of  human  body,  63,  64,  66. 
Centigrade  thermometer,  15. 
Central  heating  plant,  19. 
Chalk,  in  making  carbon  dioxide,  55. 
Charcoal,  as  a  filter,  57. 

commercially,  57. 

preparation,  57-58. 
Chemical  action,  and  electricity,  307, 
315-317. 

and  light,  126,  127. 
Chemistry,  in  daily  life,  218,  219. 
Chills,  38. 
Chloride  of  lime,  in  bleaching,  240. 

disinfectant,  251. 
Chlorine,  and  hydrogen,  239. 

effect  upon  human  body,  239. 

in  bleaching,  238-240. 

influence  of  light  upon,  126. 

presence  in  salt,  227. 
Circuit,  electric,  321. 

local,  in  telegraph,  325-326. 
City  water  supply,  206-212. 
Clarinet,  297. 
Cleaning  of  material,  226,  243. 


INDEX 


355 


Climate,    influenced   by   presence    of 

water,  29,  40. 

Clover,  nitrogen  producers,  263. 
Coal,  30. 
Coal  gas,  150,  151. 

by-products,  152. 
Coal  oil,  149,  150. 
Coal  tar  dyes,  152,  218,  245. 
Cogwheels,  170. 
Coil,  current-bearing,  320. 

magnetic  field  about,  331-333. 
Coke,  152. 
Cold  storage,  97. 
Color,  134-141. 

and  heat,  142,  143. 

influenced  by  light,  137. 

of  opaque  bodies,  130,  137. 

of  transparent  bodies,  135,  136. 
Color  blindness,  140,  141. 

designs  in  cloth,  248,  249. 
Colors,  compound,  138,  139. 

essential,  139-140. 

primary,  135. 

simple,  138. 

spectrum,  134-135. 

variety  in  dyeing,  247,  248 
Combustion,  heat  of,  45. 

spontaneous,  52. 
Commutator,  335. 
Compass,  328. 
Compound  colors,  138,  139. 
Compound  machine,  171. 
Compound  substances,  103. 
Compression  of  air,  91,  92. 

cause  of  heat,  96. 
Compression  pumps,  201,  205. 
Concave  lens,  118. 
Condensation,  33. 

heat  set  free,  40. 
Conduction  of  heat,  25. 
Conductivity  metals,  321. 
Conductors,  electric,  321,  322. 
Conservation,  of  energy,  58,  59. 

of  matter,  58,  59. 
Convection,  24,  25. 
Convex  lens,  118. 
Cooling,  by  evaporation,  35-36. 

by  expansion,  97. 
Copper,  in  electric  cell,  307. 
Core,  iron,  319. 

Corn,  bleached  with  sulphurous  acid, 
242. 


Cotton,  mercerized,  218. 

bleaching,  241. 

dyeing,  245-247. 
Cough  sirup,  258. 
Crane,  compound  machine,  172. 
Cream  of  tartar,  229. 
Creosote  oil,  254. 
Crude  petroleum,  149,  150. 
Current,  electric,  306,  312. 

alternating,  349. 

induced,  346-347. 

measurement  of,  340. 

resistance,  312,  343,  345. 

strength,  339,  340,  344. 

Dams,  214-216. 

Decay,  49. 

Decomposition  of  soil  by  water,  70-74. 

Degrees,  Fahrenheit  and  Centigrade, 

15. 

Density,  11. 
Designs  in  cloth,  printed,  248,  249. 

woven,  249. 

Developer  in  photography,  128. 
Dew,  36,  37. 
Dew  point,  38. 
Diarrhea,  251. 
Diet,  62,  66. 

economy  on  table,  66-69. 
Discord,  reason  for,  271. 
Disease,  and  surface  water,  76. 

relation  of  light  to,  131-132. 
Disease  disinfectants,  250, 251,  252. 
Distillation,  34-35. 

in  commerce,  35. 

of  petroleum,  149-150. 

of  soft  coal,  150. 

of  water,  34,  35,  77. 
Diving  suits,  204. 
Door  bells,  319-321. 
Drainage,  of  land,  194,  195. 

sewage,  196,  198,  199,  201. 
Drilled  well,  199. 
Drinking  water,  75-77. 

in  camping,  195-196. 

and  rural  supplies,  198,  201. 
Driven  well,  196-197. 
Drought,  217. 
Drugs,  255,  260. 
Dry  cell,  312. 
Dyeing,  244-249. 

color  designs,  248. 


356 


INDEX 


Dyeing,  direct,  245. 

home,  247. 

indirect,  247. 

variety  of  color,  247. 
Dyes,  218,  244,  245. 
Dynamo,  346. 

alternating  current,  349. 

source  of  energy,  346-347. 

Ear,  in  man,  301-303. 

care  of,  303. 

Earth,  conductor  of  electricity,  326. 
Echo,  277. 

Economy  in  buying  food,  66-69. 
Effort,  muscular,  155,  160. 
Electric,  battery,  311. 

bell,  319-321. 

bread  toasters,  314. 

conductors     and     non-conductors, 
321-322. 

cost  of,  energy,  352. 

current,  306,  312. 

flatiron,  313. 

heating  pad,  314. 

lights,  314. 

street  cars,  337. 
Electricity,  heat,  312-315,  339. 

as  a  magnet,  319,  331-333. 

practical  uses  of,  312-317. 
Electrodes,  of  cell,  308. 
Electrolytic  metals,  317. 
Electromagnets,  319. 
Electromotive  force,  308. 

unit  of,  344. 
Electroplating,  315. 
Electrotyping,  317. 
Elements,  102-103. 
Emulsion,  224. 
Energy,  conservation  of,  58,  59. 

transformations  of,  58,  59. 
Engine,  steam,  183-185. 

gas,  185-186. 

horse  power,  173. 
Erosion,  73-74. 
Essential  colors,  139-140. 
Evaporation,  35-39. 

cooling  effect,  35-36. 

effect  of  temperature  on,  35,  36. 

effect  of  air  on,  38. 

freezing  by,  98. 

heat  absorbed,  36. 

of  perspiration,  38. 


Expansion,  of  air,  10,  11. 

cooling  effect  of,  97. 

disadvantage    and    advantage    of, 
11-13. 

of  liquids,  9-11. 

of  solids,  10,  11. 

of  water,  9,  10,  11,  12. 
Eye,  122-125. 

headache,  124,  125. 

how  focused,  122,  123. 

nearsighted  and  farsighted,  123. 

strain,  125. 

Fahrenheit  thermometer,  15. 
Fats,  65. 

in  soap  making,  223. 
Fermentation,  232-236. 

by  yeast,  234-236. 
Ferric  compounds,  248. 
Fertilizers,  262-265. 

nitrogen,  262. 

phosphorus,  263,  264. 

potash,  263-265. 
Field  magnet,  336. 
Filings,  iron,  329. 
Film,  photographic,  129. 
Filter,  charcoal,  57. 
Filtering  water,  77. 
Fire,  9. 

and  oxygen,  45,  47. 

and  tinder  box,  47. 

making  of,  51. 

primitive  production  of,  47. 

produced  by  friction,  47. 

spontaneous  combustion,  52. 

sores  and  burns,  52-53. 

extinguisher,  55,  56. 
Fireless  cooker,  25,  26. 
Fireplaces,  17,  18. 
Fixing,  in  photography,  128. 
Flame,  hydrogen,  80. 
Flood,  Johnstown,  214,  215. 

relation  to  forests,  217. 
Flour,  self-raising,  231. 
Flume,  177. 
Flute,  297. 
Focal  length,  118. 
Focus,  of  lens,  118. 
Fog,  37. 
Food,  60-69. 

carbohydrates,  64,  65. 

economy  in  buying,  66-69. 


INDEX 


357 


fats,  65. 

fuel  value  of,  60-62. 

need  of,  63,  64. 

preservatives,  252. 

proteids,  66. 

value,  67. 

waste,  60. 

water  in,  75. 
Foot  pound,  172. 
Force  and  motion,  156,  157. 

and  work,  156,  157. 

magnetic  lines  of,  329-331,  334. 

muscular,  155,  160. 
Force  pumps,  192,  193. 
Forests  and  water  supply,  216-217. 
Forging  of  iron,  40,  41. 
Formaldehyde,  253. 
Freezing,  effect  of  salt,  44. 

effect  on  ground  and  rocks,  42. 

expansion  of  water  on,  41. 

ice  cream  freezer,  44. 
Frequency  in  music,  273,  275. 
Fresh  air,  22-24,  49. 

amount  consumed  by  gas  burner, 
22. 

and  health,  49,  50. 

in  underground  work,  202. 

in  work  under  water,  203-205. 
Friction,  173,  174. 

losses  by,  174,  210. 

source  of  heat  and  fire,  47. 
Frost,  36,  37. 

Fruit,    canned,    bleached    with    sul- 
phurous acid,  242. 

colored  with  coal  tar  dyes,  253. 
Fuel  value  of  foods,  60-62. 

table  of  fuel  values,  67. 
Fulcrum,  159,  160. 
Fumigation,  251. 
Fundamental  tone,  290,  291,  292. 
Furnace,  hot  air,  19. 
Fuse,  340. 
Fusion,  heat  of,  40. 

• 

Galvanometer,  341. 
Gas,  acetylene,  152,  153. 

and  unburned  carbon,  151. 

coal,  151,  152. 

effect  of  heat  on  volume,  96,  97. 

effect  of  pressure  on  volume,  95—96. 

engine,  185-186. 

for  cooking,  151,  152. 


illuminating,  92,  93,  150,  151. 

liquefaction,  97,  98. 

meter,  93,  94. 

natural,  152. 
Gasolene,  149,  150. 

as  cleaning  agent,  227,  243. 

in  gas  engine,  185,  186. 
Gauge,  pressure,  92-94. 
Gelatin,  plate  and  film,  129. 
Glass,  kinds  of,  119. 

molding  of,  40. 

non-conductor,  321. 
Grape  juice,  fermented    with    millet, 

233. 

Gravity  cell,  309,  310. 
Grease,  and  lye,  221. 

and  soap  making,  223. 
Gulf  Stream,  24. 

Hard  water,  and  soap,  225. 
Harp,  295. 
Headache,  124,  125. 

powders,  259. 

Health,  effect  of  diet,  62,  64. 
Heat,  9. 

absorbed  in  boiling,  31—32. 

and  disease  germs,  250. 

and  food,  252. 

and  friction,  47. 

and  light,  142,  147. 

and  oxidation,  45,  48,  49. 

and  wave  motion,  145-147. 

conduction,  25. 

convection,  24,  25. 

from  burning  hydrogen,  80. 

from  electricity,  312-315,  339. 

needed  to  melt  substances,  39. 

of  fusion,  40. 

of  vaporization,  32. 

produced  by  compression,  96. 

relation  of  water  to  weather,  29, 
40. 

set  free  by  freezing  water,  40. 

sources  of,  29-30. 

specific,  28-29. 

temperature,  27. 

unit  of,  27,  28. 
Heating    effect    of    electric    current, 

312-315. 

Heating  of  buildings:  central  heating 
plant,  19. 

fireplaces,  17-18. 


358 


INDEX 


Heating,  furnaces,  19. 

hot  water,  19-22. 
Helix,  318. 

Horse  power,  173,  351. 
Hot  water  heating,  19-22. 
Hues   primary,  135. 
Humidity,  38. 

proper  percentage  for  health  and 

comfort,  38,  39. 
Humus,  216,  217. 
Hydrocarbons,  149. 
Hydrochloric  acid,  composition,  227. 

in  bleaching,  241. 

to  make  a  salt,  227. 

to  make  carbon  dioxide,  55. 

to  make  chlorine,  238. 

to  make  hydrogen,  80. 
Hydrogen,  65,  66. 

and  chlorine,  239. 

and  water,  79. 

chemical  conduct,  126-127. 

flame,  80. 

in  voltaic  cell,  307. 

peroxide,  53,  252. 

preparation,  80. 

to  liquefy,  97. 

Ice,  lighter  than  water,  42. 

manufacture  of,  98,  99. 
Ice  cream  freezers,  44. 
Illuminating    gas,     manufacture    of, 
150,  151. 

measurement     of     quantity     con- 
sumed, 93,  94. 

test  of  pressure,  92,  93. 
Illumination,  intensity  of,  105,  106. 
Image,  in  mirror,  108,  111. 
Incandescent  lighting,  107,  314. 
Incidence,  angle  of,  110. 
Inclined  plane,  162-166. 
'      screw,  166. 

wedge,  16C. 
Indigo,  218. 

Induced  current,  346-347. 
Ink  spots,  removal  of,  243. 
Insoluble  substances,  71. 
Insulators,  electric,  324. 
Intensity,  of  light,  105-107. 

of  sound,  270-271. 
Interval,  in  musical  scale,  283. 
Iron,  forging,  41. 

filings,  329. 


galvanizing,  49. 

oxidation  of,  48. 

Irrigation,  193-194. 

Isobaric  lines,  SS,  91. 
Isothermal  lines,  89,  91. 

Johnstown  flood,  214,  215. 

Kerosene,  149,  150. 
Kilowatt,  351. 

Lactic  acid,  230. 
Leaves,  132,  262. 
Lens,  117-121. 

concave,  118. 

converging,  118. 

crystalline,  of  eye,  122. 

focal  length,  118. 

material,  119. 

refractive  power,  119. 
Lever,  158-162. 

examples,  160-162. 

fulcrum,  159,  160. 
Life,  and  carbon  dioxide,  54. 

and  nitrogen,  261. 

and  oxygen,  49,  54. 
Lifting  pumps,  189-192. 
Light,  absorption,  135-138. 

and  heat,  142-147. 

a  wave  motion,  145-147. 

bent  rays,  113,  114. 

chemical  action,  126-127. 

disease.  131-132. 

essential  to  life,  131,  132. 

fading  illumination,  105,  106. 

influence  on  color,  134. 

reflection  of,  109-112. 

refraction  of,  113-125. 

travels  in  a  straight  line,  108. 

white,  composed  of  colors,  134. 
Lighting,  artificial,  148-153. 
Lime,  chloride  of,  240,  251. 
Limewater,  220. 

anfl  carbon  dioxide,  228. 
Linen,  bleaching,  241. 

dyeing,  245-247.    . 
Lines,  of  force,  329-331,  334. 
•     isobaric,  88,  91. 

isothermal,  89,  91. 
Liquefaction  of  gases,  97,  98. 
Liquid  air,  98. 
Liquid  soap,  223,  224. 


INDEX 


359 


Litmus,  action  of  acids,  220. 

action  of  bases,  221,  222. 

action  of  neutral  substance,  222. 
Logwood  dyes,  245,  247,  248. 
Los  Angeles  aqueduct,  211. 
Lye,  221,  222. 

Machines,  compound,  171. 

inclined  plane,  162-166. 

lever,  158-162. 

pulley,  166-169. 

wheel  and  axle,  169-171. 
Madder,  for  dyes,  245. 
Magnet,  328. 

electro-,  319. 

field  of,  329-331. 

lines  of  force  about,  329-331. 

poles  of,  330-332. 

properties  of  electricity,  318. 
Magnetic,  needle,  328. 

poles,  3297-331. 

Magnifying      power,      of      a      lens, 
115. 

of  a  microscope,  115. 

of  a  telescope,  115. 
Mammoth  Cave  of  Kentucky,  71. 
Manganese  dioxide,  46. 

chlorine  made  from,  238. 

oxygen  made  from,  46. 
Marble,  for  carbon  dioxide,  55. 
Matches,  47. 

safety,  47-48. 
Matching  colors,  137. 
Matter,  conservation  of,  58,  59. 
Meat,  66. 

preservation  of,  253. 
Mechanical  devices,  154,  155. 
Melting,  39,  40. 
Melting  point,  40. 
Melting  substances  without  a  definite 

melting  point,  40. 
Mercerized  cotton,  218. 
Mercury,  barometer,  84. 

thermometer,  14-17. 
Metals,  electroplating,  317. 

preservation  by  paint,  253-254. 

veins   deposited   by   precipitation, 
72,  73. 

welding,  315. 
Meter,  gas,  93,  94. 
Microorganisms,  132,  133. 
Microscope,  115. 


Milk,  boiling  point,  32. 

Pasteurized,  250. 
Minerals,  in  foods,  62,  63. 

in  water,  70,  71. 
Mirrors,  108-112. 

distance  of  image  behind  mirror,  111. 

distance  of  object  in  front  of  mirror, 
111. 

image  a  duplicate  of  object,  111. 
Molding  of  glass,  40. 
Molecule,  100-103. 
Mordants,  247,  248,  249. 
Morphine,  257. 
Morse,  telegraphic  code,  324. 
Motion,  in  sound,  266,  278,  280. 

in  work,  156. 
Motor,  electric,  336. 

principle  of,  333. 

street  car,  337. 
Mouth  breathing,  50. 

cause  of,  51. 

Movable  pulley,  167,  168. 
Music,  278. 
Musical  instruments,  percussion,  299. 

stringed,  284-295. 

wind,  295,  299. 
Musical  scale,  282. 

Naphtha  in  gas  engines,  185. 
Naphthalene,  152. 
Narcotics,  255. 
Natural  gas,  152. 
Needle,  magnetic,  328. 
Negative,  electrode,  308. 

photographic,  130. 
Neutral  substance,  222. 

and  litmus,  222. 
Neutralization,  222. 
Niagara  Falls,  176. 
Nitrogen,  66. 

and  bacteria,  263. 

and  plant  life,  261. 

in  atmosphere,  261. 

in  fertilizer,  262-265. 

in  food,  66. 

preparation  of,  261. 

properties  of,  261. 
Noise  in  music,  280. 
Non-conductors,  of   electricity,    321- 
322. 

of  heat,  25. 
Nutcracker,  as  a  lever,  162. 


INDEX 


Oboe,  297. 

Octave,  284. 

Odors,  101. 

Ohm,  unit  of  resistance,  345. 

Oil,  gasoline,  149,  150. 

kerosene,  149,  150. 

lubricating,  174. 

olive,  16. 

Orchestra  grouping,  299. 
Ore,  72. 

Organ  pipes,  297. 
Overtones,  290-293. 
Oxalic  acid,  247,  248. 
Oxidation,  45-59. 

and  decay,  49. 

heat  the  result  of,  49-52. 

in  human  body,  49,  53. 

of  iron,  48. 
Oxygen,  66. 

and  bleaching,  239. 

and  combustion,  45. 

and  food,  66. 

and  plants,  55. 

and  the  human  body,  50. 

and  water,  79,  80. 

in  the  atmosphere,  45. 

preparation  of,  46. 

Paint,  as  wood  and  metal  preserva- 
tives, 253,  254. 

removal  of  stains,  243. 
Paper  making,  219. 
Paraffin,  150,  321. 
Pasteurized  milk,  250. 
Patent  medicines,  257-260. 
Peas,  sources  of  nitrogen,  263. 
Pelton  wheel,  177. 
Percussion  instruments,  299. 
Period  of  a  body,  273. 
Peroxide  of  hydrogen,  53,  252. 
Petrolatum,  150. 
Petroleum,  149,  150. 
Phonograph,  303-305. 
Phosphorus,  in  fertilizer,  263,  264. 

in  making  nitrogen,  261. 

in  matches,  47,  48. 

poisoning  by,  47. 
Photography,  127-131. 
Photometer,  107. 
Pianos,  284-292. 
Pin  wheel,  181. 
Pitch  of  sound,  280,  281. 


cause  of,  282. 

in  wind  instruments,  296-299. 
Plane,  inclined,  162-166. 
Plants,  and  atmosphere,  55. 

and  light,  131-132. 

and  nitrogen,  261. 

Plate  developing,  photographic,  128. 
Pneumatic  dispatch  tube,  205. 
Poles,  magnetic,  330-332. 

of  cell,  308. 

Positive  electrode,  308. 
Potash,  in  fertilizer,  263-265. 
Potassium      chlorate     and     oxygen, 
46. 

permanganate,  100. 

tartrate  and  Rochelle  salt,  227. 
Power,  candle,  105-107. 

electric,  351. 

horse,  173,  351. 

sources  of,  174,  175,  185. 

transmission  by  belts,  171. 

water,  176-180. 
Precipitation,  72,  73. 
Preservatives,  food,  252. 

wood  and  metal,  253-254. 
Pressure,  atmospheric,  82-86. 

calculation  of  atmospheric,  83,  84. 

calculation  of  gas,  92,  93. 

calculation  of  water,  94. 

gauge,  92-94. 

of  illuminating  gas,  93. 

relation  of  pressure  of  gas   to  vol- 
ume, 95,  96. 

water  pressure,  208-211,  214-216. 

within  the  body,  86. 
Primary  colors,  135. 
Print,  photographic,  131. 
Printing,  color  designs  in  cloth,  248, 
249. 

electrotype,  317. 
Prisms,  135. 

refraction  through,  117. 
Proteids,  66. 
Pulleys,  166-169. 

applications  of,  169. 
Pump,  187-205. 

air,  201-205. 

force,  192,  193. 

lifting,  189-192. 
Pupil  of  the  eye,  122. 
Pure  food  laws,  bleaching,  242. 

preservatives,  252. 


INDEX 


361 


Purification  of  water,  77,  196. 
Push  button,  321. 

Radiator,  19-21. 

Railroads,  grading  of,  165-166. 

Rain,  36,  37. 

Rainbow,  134. 

Rain  water,  225. 

Reflection,  angle  of,  110. 

of  light,  109-112. 

of  sound,  278,  279. 
Refraction,  angle  of,  114. 

by  atmosphere,  114. 

of  light,  113. 

uses  of,  115-116. 
Relay,  telegraph,  325. 
Reservoir,  214. 

artificial,  211. 

construction  of,  214-216. 

natural,  211. 
Resistance,  electrical,  312. 

internal,  of  cell,  343. 

unit  of,  345. 
Resonance,  276. 

River,  volume  and  value  of,  180. 
Roads,  application  of  inclined  plane 

to,  165-166. 
Rochelle  salt,  227,  231. 
Rocks,    effect   of   freezing  water   on, 
42-43. 

water  as  a  solvent,  71. 
Rosin,  obtained  by  distillation,  35. 

Safety  matches,  47-48. 
Salicylic  acid,  253. 
Salt,  227-228. 
Salts,  227. 

general  properties,  227. 

in  ocean,  227. 

smelling,  222. 
Saturation  of  air,  37. 
Scale,  musical,  282. 
Screw,  and  inclined  plane,  166. 
Seaweed,  265. 
Seidlitz  powder,  231. 
Self-raising  flour,  231. 
Sewage,  disposition  of,  198-199. 

of  camps,  196. 

source  of  revenue,  201. 
Sewer  gas,  57. 
Silk,  bleaching,  241. 

dyeing,  245-247. 


Silver  chloride,  127-131. 
Simple  colors,  138. 
Simple  substances,  103. 
Siren,  280. 
Smelling  salts,  222. 
Snow,  36-37. 
Soap,  222-224. 

and  hard  water,  225. 

liquid,  223-224. 

preparation,  223. 
Soda,  baking,  227,  228-229. 

benzoate,  253. 

caustic,  218,  222,  223,  227. 

washing,  225,-  226,  229. 
Soda  mints,  231. 

Sodium,   bicarbonate,    56,   227,   228, 
230-231. 

carbonate,  228. 

chloride,  228. 

Soil,  deposited  by  streams,  73. 
Solenoid,  318. 
Solution,  70. 
Soothing  sirup,  258. 
Sound,  and  motion,  266,  278. 

musical,  278. 

nature  of,  266. 

reflection,  277. 

speed  of,  271-272. 

transmission  of,  267—271. 

velocity  of,  271-272. 

waves,  272-274. 
Sounder,  telegraph,  324. 
Sounding  board,  277. 
Sour  milk  in  cooking,  230. 
Specific  heat,  28-29. 
Spectrum,  134-135. 
Speed,  of  sound,  271,  272. 
Spontaneous  combustion,  52. 
Stains,  removal  of,  226,  243. 
Standpipes,  212. 
Starch,  65. 
Steam,  and  work,  183-184. 

engine,  183-185. 

heat  of  vaporization,  32. 

heating  by,  33. 

turbine,  183-184. 
Steel,  forging  and  annealing,  16. 
Stoves,  18-19. 
Streams,  carriers  of  mud,  73. 

volume  of,  179-180. 
Street  cars,  electric,  337. 
Stringed  instruments,  284-295. 


362 


INDEX 


Strings,  vibrating,  286-290. 
Sugar,  16,  65. 

fermented  by  yeast,  234. 
Sulphur,  66. 

as  disinfectant,  251. 

in  making  sulphurous  acid,  242. 
Sulphuric  acid,  in  bleaching,  240,  241. 

in  fire  extinguisher,  55. 

in  making  of  hydrogen,  80. 
•   in  voltaic  cell,  307. 
Sulphurous  acid,  in  bleaching,  242. 

preparation,  242. 
Sun,  energy  derived  from,  143—144. 

source  of  heat,  29-30. 
Sunlight,  135. 

and  bacteria,  133. 

and  chemical  action,  126-127. 
Sympathetic  vibrations,  274-277. 

Tallow,  105,  148. 
Tartar,  cream  of,  229. 
Telegraph,  322. 

long  distance,  327. 

relay,  325. 

sounder,  324. 
Telephone,  350-351. 
Temperature,  13-14. 

as  measurement    of    heat   present, 
27. 

in  detecting  adulterants,  17. 

in  forging  steel,  16. 

in  making  sirups,  16. 

measurement  of,  14-15. 
Thermometer,  14-17. 

Centigrade,  15. 

Fahrenheit,  15. 
Tinder  box,  47. 
Transmission,  of  light,  145-147. 

of  sound,  267-271. 
Tuning  fork,  266,  273,  278,  290. 
Turbine,  steam,  183. 

water,  178. 
Turpentine,  and  grease,  226. 

by  distillation,  35. 

Unleavened  bread,  233. 

Vacuum,  sound  in,  268. 
Vapor,  in  atmosphere,  36-38. 
Vaporization,  heat  of,  32. 
Varnish,  on  candies,  253. 
Vegetable  matter,  and  coal,  30. 


and  gas,  30. 

and  oil,  30. 

.Veins,  formation  in  rock,  72-73. 
Velocity,  of  sound,  271-272. 
Ventilation,  21-24,  54. 

need  of,  38. 
Vibration,  of  strings,  286-290. 

sympathetic,  274-277. 
Viola,  295. 
Violin,  295. 
Violoncello,  295. 
Vocal  cords,  300. 
Voice,  300. 
Volt,  344. 
Voltage,  345. 

Voltaic  cell,  306-308,  310. 
Voltmeter,  344. 
Volume,  of  a  stream,  179-180. 

relation  of  pressure  of  a  gas,  95- 
96. 

Washing  powders,  224-226. 

soda,  229. 
Water,  action  in  nature,  70-74. 

amount  used  daily  per  person,  181. 

and  hydrogen,  79. 

and  oxygen,  79,  80. 

as  solvent,  70-71. 

boiling,  77. 

boiling  point,  15. 

composition,  79-80. 

condensation,  33. 

dams  and  reservoirs,  214-216. 

density,  11. 

distilled,  34,  77. 

drinking,  75-77,  195-201. 

electrolysis,  79-80. 

evaporation,  33-34. 

expansion,  9-10,  41-42. 

filtration,  77. 

freezing,  40—41. 

hard,  225. 

heat  of  fusion,  40. 

impurities,  76-77. 

in  atmosphere,  36—38. 

in  food,  75. 

in  human  body,  75. 

in  vegetables,  75. 

influence  on  climate,  29,  40. 

irrigation,  193-194. 

minerals  in,  70-71. 

ocean,  265. 


INDEX 


363 


power,  176-180. 

precipitates,  72,  73. 

pressure,  208-211,  214-216. 

purification,  77. 

rain,  225. 

running,  value  of,  178-180. 

source  of,  78. 

steam,  32. 

waves,  145-147. 

weight,  208-209,  215. 

wells,  195-201. 

wheels,  176-180. 

work  under,  203-205. 
Water  supply,  and  forests,  216-217. 

cost,  212-214. 

of  city,  206-212,  217. 
Watt,  351. 
Waves,  heat,  145-147. 

light,  145-147. 

sound,  268,  272-274. 

water,  145-147. 
Weather,  bureau,  87-91. 

forecasts,  38-39,  86-88. 

relation  of  water  to,  29,  40.. 
Weather  maps,  89-91. 
Wedge,  and  inclined  plane,  166. 
Weight,  of  air,  86. 

of  water,  208-209,  215. 
Welding,  by  electricity,  315. 
Wells,  195-201. 

drilled,  199. 

driven,  196-197. 


Wheel  and  axle,  169-171. 

cogwheels,  170. 

windlass,  169. 

Wheelbarrow  as  lever,  160-161. 
White  light,  nature  of,  135. 
Wind  instruments,  297-301. 
Windlass,  169. 
Windmill,  174-175,  180-182. 
Winds,  24. 
Wine,  232,  234. 
Wood,  as  source  of  charcoal,  58. 

ashes  in  soap  making,  223. 

in  paper  making,  219. 

preservation,  253-254. 
Wool,  bleaching,  241. 

dyeing,  245-247. 
Work,  156-186. 

and  steam,  183-184. 

and  water,  176-180. 

conservation,  174-175. 

formula,  157. 

machines,  157-175. 

unit  of,  172-173. 

waste,  173. 
Woven  designs  in  cloth,  249. 

Yeast,  234-236. 
wild,  235-236. 

Zinc,  in  galvanizing  iron,  49. 
in  making  hydrogen,  80. 
in  voltaic  cell,  307-308. 


COMPOSITION-RHETORIC 

1 1. 00 

By  STRATTON  D.  BROOKS,  Superintendent  of  Schools, 
Boston,  Mass.,  and  MARIETTA  HUBBARD,  for- 
merly English  Department,  High  School,  La  Salle,  111. 


THE  fundamental  aim  of  this  volume  is  to  enable  pupils 
to  express  their  thoughts  freely,   clearly,   and  forcibly. 
At  the  same   time  it   is    designed    to   cultivate   literary 
appreciation,  and    to  develop  some  knowledge  of  rhetorical 
theory.      The  work  follows  closely   the  requirements  of  the 
College  Entrance  Examination  Board,  and  of  the  New  York 
State  Education  Department. 

^[  In  Part  One  are  given  the  elements  of  description,  narra- 
tion, exposition,  and  argument ;  also  special  chapters  on  letter- 
writing  and  poetry.  A  more  complete  and  comprehensive 
treatment  of  the  four  forms  of  discourse  already  discussed  is 
furnished  in  Part  Two.  In  each  part  is  presented  a  series  of 
themes  covering  these  subjects,  the  purpose  being  to  give  the 
pupil  inspiration,  and  that  confidence  in  himself  which  comes 
from  the  frequent  repetition  of  an  act.  A  single  new  princi- 
ple is  introduced  into  each  theme,  and  this  is  developed  in  the 
text,  and  illustrated  by  carefully  selected  examples.  These 
principles  are  referred  to  again  and  again  as  the  subject 
grows. 

^j  The  pupils  are  taught  how  to  correct  their  own  errors, 
and  also  how  to  get  the  main  thought  in  preparing  their 
lessons.  Careful  coordination  with  the  study  of  literature 
and  with  other  school  studies  is  made  throughout  the  book. 
^|  The  modern  character  of  the  illustrative  extracts  can  not  fail 
to  interest  every  boy  and  girl.  Concise  summaries  are  given 
following  the  treatment  of  the  various  forms  of  discourse,  and 
toward  the  end  of  the  book  there  is  a  very  comprehensive  and 
compact  summary  of  grammatical  principles.  More  than  usual 
attention  is  devoted  to  the  treatment  of  argument.  The  ap- 
pendix contains  the  elements  of  form,  the  figures  of  speech,  etc. 


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A  HISTORY  OF  AMERICAN 
LITERATURE 

By  REUBEN  POST  HALLECK,  M.A.,  Principal,  Male 
High  School,  Louisville,  Ky. 


A  COMPANION  volume  to  the  author's  History  of  Eng- 
_/""V  lish  Literature.  It  describes  the  greatest  achievements 
in  American  literature  from  colonial  times  to  the  pres- 
ent, placing  emphasis  not  only  upon  men,  but  also  upon  liter- 
ary movements,  the  causes  of  which  are  thoroughly  investi- 
gated. Further,  the  relation  of  each  period  of  American 
literature  to  the  corresponding  epoch  of  English  literature 
has  been  carefully  brought  out — and  each  period  is  illumin- 
ated by  a  brief  survey  of  its  history. 

^j  The  seven  chapters  of  the  book  treat  in  succession  of 
Colonial  Literature,  The  Emergence  of  a  Nation  (1754.- 
1809),  the  New  York  Group,*  The  New  England  Group, 
Southern  Literature,  Western  Literature,  and  the  Eastern 
Realists.  To  these  are  added  a  supplementary  list  of  less 
important  authors  and  their  chief  works,  as  well  as  A  Glance 
Backward,  which  emphasizes  in  brief  compass  the  most  im- 
portant truths  taught  by  American  literature. 
5[  At  the  end  of  each  chapter  is  a  summary  which  helps  to 
fix  the  period  in  mind  by  briefly  reviewing  the  most  significant 
achievements.  This  is  followed  by  extensive  historical  and 
literary  references  for  further  study,  by  a  very  helpful  list  of 
suggested  readings,  and  by  questions  and  suggestions,  designed 
to  stimulate  the  student's  interest  and  enthusiasm,  and  to  lead 
him  to  investigate  for  himself  the  remarkable  literary  record  of 
American  spirituality,  individuality,  initiative,  and  democratic 
aspiration  and  accomplishment. 

^j  While  within  the  comprehension  ot  secondary  pupils,  the 
treatment  is  sufficiently  full  and  suggestive  for  a  systematic, 
progressive  study  of  American  literature. 


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Burke's  Conciliation  with  the  American  Colonies  (Clark)         .    .          .20 

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Byron's  Childe  Harold  (Canto  IV),  Prisoner  of  Chillon,  Mazeppa, 

and  other  Selections  (Venable)       .20 

Carlyle's  Essay  on  Burns  (Miller) .20 

Chaucer's  Prologue  and  Knighte's  Tale  (Van  Dyke) 20 

Coleridge's  Ancient  Mariner  (Garrigues) .20 

Cooper's  Pilot  (Watrous).      Double  number .40 

Defoe's  History  of  the  Plague  in  London  (Syle) 20 

Robinson  Crusoe  (Stephens) .20 

Dickens's  Tale  of  Two  Cities  (Pearce).      Double  number    .    .    .          .40 

Dryden's  Palamon  and  Arcite  (Bates) .20 

Emerson's  Essays.     Selections  (Smith) .20 

Franklin's  Autobiography  (Reid) .20 

George  Eliot's  Silas  Marner  (McKitrick) .20 

Goldsmith's  Vicar  of  Wakefield  (Hansen) .20 

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Irving's  Sketch  Book — Selections  (St.  John) .20 

Tales  of  a  Traveler  (Rutland).      Double  number      .....          .40 

Macaulay's  Essay  on  Addison  (Matthews) .20 

Essay  on  Milton    (Mead) .20 

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Shakespeare's  As  You  Like  It  (North) 20 

Hamlet  (Shower) 20 

Julius  Caesar  (Baker) 20 

Macbeth  (Livengood) .20 

Merchant  of  Venice  (Blakely) 20 

Midsummer-Night's  Dream  (Haney) 20 

Twelfth  Night  (Weld) 20 

Tennyson's  Idylls  of  the  King.      Selections  (Willard) .20 

Princess  (Shryock) 20 

Thackeray's  Henry  Esmond  (Bissell).      Triple  number      .    ,     ,    .  60 

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From  the  earliest  records  to  Charlemagne.  By  ARTHUR 
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ESSENTIALS  IN   MEDIEVAL  AND   MODERN 
HISTORY |i.5o 

From  Charlemagne  to  the  present  day.  By  SAMUEL 
BANNISTER  HARDING,  Ph.D.,  Professor  of  Euro- 
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ESSENTIALS  IN  ENGLISH  HISTORY    .     .     $1.50 

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From  the  discovery  to  the  present  day.  By  ALBERT 
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Harvard  University. 


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quired   by  the   College  Entrance   Examination   Board, 
and  by  the  New  York  State  Education  Department. 
Each  volume  is  designed  for  one  year's  work.      Each  of  the 
writers  is  a  trained  historical  scholar,  familiar  with  the  con- 
ditions and  needs  of  secondary  schools. 

*|j  The  effort  has  been  to  deal  only  with  the  things  which 
are  typical  and  characteristic;  to  avoid  names  and  details 
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with  the  forces  which  have  really  directed  and  governed  man- 
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to  the  movements  of  sovereigns  and  political  leaders. 
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but  useful  sets  of  bibliographies  and  suggestive  questions. 
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ESSENTIALS  OF  LATIN  FOR 
BEGINNERS 

By  HENRY  C.  PEARSON,  Horace  Mann  School, 
Teachers  College,  New  York.  Author  of  Latin  Prose 
Composition,  Greek  Prose  Composition 

^0.90 


THIS  book  is  designed  to  prepare  pupils  in  a  thorough 
fashion  to  read  Caesar's  Gallic  War.  It  contains 
seventy  lessons,  including  ten  that  are  devoted  exclu- 
sively to  reading,  and  six  supplementary  lessons.  The  first 
seventy  lessons  contain  the  minimum  of  what  a  pupil  should 
know  before  he  is  ready  to  read  Latin  with  any  degree  of 
intelligence  and  satisfaction.  The  supplementary  lessons  deal 
largely  with  certain  principles  of  syntax  that  may  be  taken  up 
or  omitted,  according  to  the  desire  of  the  teacher. 
^|  The  vocabularies  have  been  carefully  selected,  and  contain, 
with  very  few  exceptions,  only  those  words  that  occur  with 
the  greatest  frequency  in  Caesar's  Gallic  War.  About  five 
hundred  words  are  %  presented  in  the  first  seventy  lessons. 
There  is  a  constant  comparison  of  English  and  Latin  usage, 
but  not  much  knowledge  of  English  grammar  on  the  part  of 
the  pupil  is  taken  for  granted.  The  more  difficult  construc- 
tions are  first  considered  from  the  English  point  of  view. 
^[  The  topics,  such  as  nouns,  adjectives,  pronouns,  and  verbs, 
are  not  treated  in  a  piecemeal  fashion,  but  four  or  five  con- 
secutive lessons  are  devoted  to  one  topic  before  passing  on  to 
another.  Sufficient  change,  however,  is  introduced  to  avoid 
monotony.  The  work  is  provided  with  dimple  reviews ;  the 
regular  exercises  review  the  vocabulary  and  constructions  of 
the  preceding  lessons,  and  these  are  supplemented  by  review 
exercises.  The  last  twenty  pages  are  devoted  to  carefully 
graded  material  for  reading,  composed  of  selections  from  Viri 
Romae  and  the  first  twenty  chapters  of  Caesar's  Gallic  War, 
Book  II.,  in  simplified  form. 


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